Upgrades, Equipment, and Skill Trees

Cross-posted on gamasutra.

As part of my continuing series on the design of our new tactical tower defense / RPG hybrid Defender's Quest, I'm going to talk a little bit about upgrade systems, equipment, and skill trees.

Before we get started, let's ask ourselves a simple question: what's the point of upgrades?

Upgrade Complete!

The way I see it, an upgrade system provides the following things*:

  1. A steady dopamine drip
  2. A way to customize your play style
  3. A chance to make interesting choices
I'm sure there's more. These are just the basics.


Dopamine drip

This one's obvious. Scheduled rewards work - upgrade systems give you a steady feeling of progression and novelty that's just around the corner. Zynga and others have been criticized for allegedly abusing this mechanic, but you would be hard pressed to find a game that doesn't take advantage of human psychology in this way.

Without bringing up that whole debate, I'll just say that scheduled rewards can be part of a balanced gaming breakfast, but if we want more than a digital skinner box, we need to add depth. That's where the next two points come in.


Let's take the most boring RPG scenario possible - you level up, and so do the monsters, at the exact same rate.

The numbers have changed, but the ratios are identical. There's no variation between the hero's abilities in relation to the monsters, so there's no reason to change strategy or do anything else that's interesting. Furthermore, if the monsters level up faster or slower than the player, the game will become too hard or too easy.

Now, let's make one small change. Let's let the player decide where the points go:

Now we've got something interesting. This player has put all of her points into "being sneaky," putting her head and shoulders above the monsters, but allowing them to catch up in "killing stuff" and "not dying." This character must change their strategy to reflect their new strengths and weaknesses - most likely sneaking around, stealing treasure, and avoiding direct combat.

Even in this oversimplified example, customization adds replay value and diverse play styles. Furthermore, deciding how to customize your character is fun in and of itself, which brings me to the next point.

Interesting Choices

I like upgrade systems where you have to pick between a few different things and can't have them all. Generally, an interesting choice is a set of options that are different, balanced, clear, and limited.

Different and Balanced

Options should be qualitatively different from one another. Here's an example from space shooters - missiles or lasers? A laser fires instantaneously, but its damage decreases with range. A missile travels slowly, but deals full damage when it reaches its target. Each of these will require a different play style.

Options should be balanced, which means that no option is strictly better than another. If one stands out above the rest, players will always pick it - there's no real choice there. 

Picking between two or three equally good but very different things creates an enjoyable tension. 

If options are not balanced and different, then the choice is not interesting. Without balance, the best option is obvious, so the lesser ones are never explored. If the options are too similar, it doesn't matter what the player picks.

Limited and Clear

Choices should consists of limited options. If there's too many to pick from, choice paralysis sets in. It's like shopping for soap - if you're like me, you don't really care what brand you get, but being faced with 36 different choices stops you in your tracks anyway. This doesn't mean there shouldn't be a lot of options - just not too many options at once. The ideal number varies with context, but generally speaking - the fewer, the better.

AAAAAAAHHH! Too many choices!

Lastly, each option should be clear. If the player doesn't know what each one will do, the choice isn't interesting, and therefore isn't fun. For example, if a skill upgrade simply says, 

+15 to Finesse 

that doesn't help me. I don't know what "Finesse" does. If instead, it says,
+15 to Finesse (Increases critical hit chance by 15%)

That's better. Depending on the game, you might even just say:
Increases critical hit chance by +15%

There's no magic formula for designing clear systems, but it's safe to assume that the first draft of your system will be too complicated, unclear, and/or paralyzing. Kleenex testing is a good way to see if your choices are working. When you get to the part where players are saying, "Well, this is good because X, but I really want Y because it does that...." you're on the right track.

Making interesting choices is fun in and of itself, apart from the strategic consequences of those choices. This is why I choose to separate "Interesting Choices" from "Customization," whereas others might consider them parts of the same thing.

A test in which you only use a fresh player who has never seen your game before, and then never use that player again (hence "kleenex"). You stand by and watch but offer no help, and don't answer any of their questions. This gives you an idea of how a fresh player would experience your game at home for the first time. See Steve Krug's Don't Make Me Think! and Rocket Surgery Made Easy for more.

Level Up!

With the basics out of the way, let's turn our attention to RPG's.

These games, at least in terms of mechanics, are about upgrades. The most common upgrade system (besides leveling up itself) is equipment. Second to that is the "skill tree" in all its various guises

There's countless others, but we'll be limiting our discussion today to just these two.


Equipment is stuff you buy with money or find as treasure. It has the following properties:
  1. Can sell it back
    You can usually only sell equipment back for a portion of its cost, but this is in sharp contrast to skill trees, where once you assign a point it's spent forever*. This takes some of the sting out of "buyer's remorse" for equipment and makes it easier to commit to a purchase decision. Not only that, but if you find loot you don't want, you can turn it into cash and buy something else.

    Generally speaking. I'm aware some games let you re-spec.

  2. Can be shared
    If several party members can equip swords, a single sword can be "handed down" a few times to other characters before it becomes obsolete. This makes equipment a general investment in the party itself, rather than a one-time investment in a single character.

  3. Concrete
    Equipment depicts real things. It's easy to imagine holding a sword in your hand, whereas an abstract concept like "+10 dexterity" just floats around in the aether. Because of its concrete nature, small efforts to make equipment seem more real, like icons, descriptions, etc, go a long way towards improving the experience because it gives our minds something to attach to. RPG's that treat equipment as nothing more than stats with a name miss out on this.

    This is all about making the equipment come alive. For example, Desktop Dungeons sparks the imagination with just a name and description for each item, because each one is unique and does something different. On the other hand Diablo's weapons feel generic despite their gorgeous illustrations and fancy titles because most of them are procedurally generated and largely disposable.

  4. Fun to get
    This follows naturally from above. It just feels good to get loot. Since equipment is concrete, it feels better to get a +10 sword rather than an abstract +10 strength boost, even though both might do the same thing.

  5. Justifies money
    Without equipment, there's less point to having money. Money is always a great reward, not only because it allows the player to choose what equipment they want, but for the same reason it feels great to get coins in Mario - humans just like getting money. Also, spending money is an opportunity for fun choices.

    Spending money becomes a more interesting decision if there's more for sale than just equipment. Common things that compete with equipment for the player's money include consumable items like potions, services like healing at the inn, and recruiting new party members.

  6. Non-cumulative
    Absent dual-wielding, you can't equip two swords to one character. This means that if you buy a pretty good sword today and a great sword tomorrow you've wasted money, since the bonuses don't stack on each other. This creates an interesting choice - should the player buy the latest equipment now, or hold out for better stuff down the road? In the first case, fighting battles is easier now, at the cost of being broke and under-equipped later. In the latter case, the player has to suffer through harder battles now in order to be richer and better-equipped later.

    In games with large numbers of characters, this effect is muted a bit because you can hand down used weapons to other characters. 

Skill Trees

When I talk about "skill trees," I'm referring to upgrade schemes where the player directs how each character unlocks new abilities and bonuses as they level up. Usually this involves assigning points earned on level-up to nodes on a branching graph. Here's some interesting features of skill trees:

  1. Tied to an individual
    Each character gets their own skill points on level up. In contrast, resources like money and equipment belong to the entire party, so outfitting one character often means neglecting another. Tying skill points to individuals lets the player upgrade each character without worrying about whether those points could be better spent elsewhere.

  2. Cumulative
    Unlike equipment, every point you spend in a character's skill tree makes them better than they were before, so there's no way to entirely "waste" a skill point. The worst a player can do is pick a less awesome choice than she could have.

  3. Abstract
    Whereas equipment represents a noun ("sword"), a skill represents a verb ("smash") or adjective ("hardiness"). This can make skill trees harder to understand at a glance than equipment. Clarity is perhaps the biggest design challenge for skill trees.

  4. Gated
    Skill trees are usually gated
    . This means that each node on the graph has some basic requirement to unlock it. Usually this means the characters has to be at a certain level, and/or has invested X points in a previous skill. This limits the player's choices early on, and expands them later as they learn more about the game. Gating serves two functions - reducing choice paralysis early on, and maintaining balance.

    Sometimes equipment systems are gated, too (ie, "you must be level 10 to equip this sword"), but it's not inherent to equipment systems in the way it is for skill trees.
Putting It Together

With all that in mind, here's how we put together our upgrade systems for Defender's Quest.

Our game is a tactical RPG with an involved tower-defense battle system and a party size in the dozens, essentially a small army. In most tower defense games, each tower of a given type is exactly the same, but in Defender's Quest each "tower" is a unique character.

Strategy and Tactics

RPGs are usually heavy on strategy and light on tactics. That is, the decisions you make outside of battle have more to do with your victory than the decisions you make inside of battle. In some RPG's, strategy is literally everything, so battles are reduced to "press A until you win" if you've prepared adequately. Despite this tedium, the strategic part (or the story) must be interesting enough for us to stick around, or this genre would have died long ago.

Tower defense games, on the other hand, are heavy on tactics and light on strategy. The battle sequences are the meat of the game, and there's usually very little to do outside of battle. Sometimes there's a simple upgrade system layered on top of things, as in Revenge of the Titans and Cursed Treasure: Don't Touch my Gems!
, but not much else tying the missions together.

In general, a strategic upgrade tends to be permanent or at least long-term, whereas a tactical upgrade is usually temporary. Leveling up in an RPG permanently increases your advantage over the enemy, whereas upgrading a tower in a TD game only helps you in that individual battle.

In Defender's Quest, we're looking to merge the long-term strategy and storytelling of traditional RPG's with the short-term tactics of Tower Defense games. This way, the joy of upgrades never ends.

Two excellent games, by the way.

Interesting Choices

The fundamental choice in Def-Q
 is "Do I want a few strong things or lots of weaker things?"  This simple choice plays out on every level of the game to create a multitude of play styles.

*In lieu of "DQ," so I don't run afoul of trademarks belonging to Square Enix and Dairy Queen


  1. Characters vs. Equipment
  2. Advanced skills vs. Simple skills
  3. Traits vs. Techniques
  4. Expensive vs. Cheap


  1. Place new vs. Boost old
  2. Expensive vs. Cheap
  3. Place defenders vs. Cast spells

Let's discuss these one by one.

Strategic Choices

Characters vs. Equipment

The first dilemma is how to spend money. Money is earned as a battle reward and can be spent on either hiring new characters or buying equipment.

There are six character classes in total, and the player gets the "hero" of that class for free through story progression.  The player can hire up to five "generic" characters of each class for money at a town. Each new character hired costs more than the last, regardless of class.

The player can earn some equipment in battle, but most of it has to be bought. Equipment consists of weapons and armor and is sold in most towns.

If the player hires a lot of characters, there will be less money for equipment, and vice versa. A large group of characters can cover more area in battle, but is far less deadly than a handful of fully-equipped defenders.

Advanced skills vs. Simple skills

There are two kinds of skills in Def-Q, techniques and traits. Techniques are moves like "double strike" that defenders use in battle. The catch is that each defender has 5 techniques, and only the first is immediately available when the defender is first placed.

The player can summon a defender for fairly cheap, but must channel additional energy to "boost" them. This increases the defender's stats and unlocks a technique. The berserker's "double strike" attack becomes available at boost level 2, for example, and "whirlwind" doesn't activate until boost level 5.

A freshly summoned defender can only use his first technique

Boosting him unlocks his advanced techniques

The advanced techniques are the most powerful, but won't be available until late in a battle. The early techniques are weaker, but are available as soon as the defender is placed. Boosting is a temporary upgrade that is lost when the battle ends, or when the defender is defeated or un-summoned.

Traits vs. Techniques

Traits are passive, always on abilities. They either confer permanent stat bonuses or add unique "flavors" to techniques. The Ranger's "range focus" trait, for example, adds a permanent boost to range, whereas "deep shot" adds the "bleed" status effect to all attacks.

Techniques increase a defender's raw power, but traits give them utility, flexibility, and customization.

Expensive vs. Cheap

Each town offers both expensive and cheap items. The player can get a bunch of cheap stuff so everyone is equipped, or invest in a small number of choice items for the party's elite.

The same goes for the skill tree. Although each skill only costs one point, the tree is gated - each skill requires the character to be at a minimum level and have three points in the connecting skill. This means that it costs more to dive down one branch of the tree than it is to explore the shallows of other branches.

The determined player can plow towards advanced skills by focusing entirely on one branch, but at the cost of having fewer, less powerful, skills overall.

Tactical Choices

Place new vs. Boost old

The simplest thing the player can do in Def-Q is place a defender on the battlefield. Each additional defender increases the area the player covers. Summoning defenders costs a small amount of energy, which varies between classes for the sake of balance.

Boosting an old defender costs much more than summoning a new one, and the price increases with boost level. Since boosting increases stats and unlocks advanced techniques, boosting an old defender is a better choice than summoning a new one in terms of simple cost-benefit. Since the battle is played on a map, however, enemies can come from all directions, and a single super-boosted defender might not be able to handle them as well as several semi-boosted ones.

Expensive vs. Cheap

Each class has a different base cost. This determines not only their summoning cost, but also their cost to boost. A ranger is much cheaper than a dragon, for instance.

Defenders vs. Spells

In addition to summoning and boosting defenders, the player can spend energy to cast spells. Spells do useful things like deal direct damage to an enemy, increase the defender's rate of attack for a few seconds, push the enemies back, and more. These abilities are useful for turning the tide of battle, getting out of a tight spot, and more.

Defenders are cheap to summon and pay for themselves as they kill monsters over time. Spells, on the other hand, cost energy and give you no return on the investment other than their immediate effect.

The Dragon Fire spell is powerful, but doesn't pay for itself

Designing Equipment

In most RPG's, each piece of equipment comes with several stats. A single sword might be +2 to attack, +1 to speed, and has a +50% chance of critical hit against trolls. Equipment with multiple stats is best for a game with a small number of characters, since this makes the choice of "what do I equip" more interesting. If one sword is strictly better than another, the choice is obvious.

In a game with many characters, however, this leads to choice paralysis and hours sunk micro-managing the entire party. In Def-Q, we decided equipment should only have one stat - weapons simply increase attack power, and armor simply increases defense.

This works well for a game with a huge party like ours. The decision is no longer "which weapon is best," but rather, "who do I give the best weapon to?" Not only that, single stat equipment allows us to design interfaces that show the entire party's situation at a glance.


One of the biggest problems games with complicated upgrade systems face is interface. Interface design is hard enough as it is, but the more complicated your system, the more screens the player has to manage. I've played RPG's where I've gotten so lost in sub-screens that I couldn't remember what I was trying to manage in the first place.

In Defender's Quest, managing both equipment and skills can be done from a single screen. Here's a quick rundown.

There are two things to do in the party screen - assign points to the skill tree, and equip weapons and armor. The currently selected character shows up on the left, with relevant stats and information. On the right are all the party members, grouped by class.

By default, the screen shows the current character's skill tree. In this mode, the question on the player's mind is, "which skills do I assign to this character," and "which other characters do I need to assign points to?" On the right, each character displays their name, level, and how close they are to leveling up. Any defenders that have skill points to spend show "X pts" in their level up bar. This lets the player tell at a glance who they need to click on to manage next.

Take a look at this cropped image of the party screen:

Click to make big
Among the player's stats are two large buttons showing the current equipment. Clicking on either of these will change the party screen from skill mode to equip mode. The equipment menu lists only equipment that the current character's class can equip, and shows a quick comparison as to which items are better and worse.

In this mode, the player's questions are "what do I equip to this character?" and, "who else needs to be equipped?" In equip mode, the other character's current equipment + bonuses are shown in lieu of experience bars and skill points. This lets the player easily scan their entire army at a glance rather then inspecting them one by one.

Click to make big

It should be easy for players to see at a glance where they've forgotten to allocate skill points, and which defenders need their equipment changed.


To sum things up, the RPG meta-game is all about upgrades. All of the systems that have come to us from previous games have specific purposes, so the proper use of them requires an understanding of what they do and how they interact with other systems.

This has been our attempt to marry both strategic and tactical upgrade systems into an engaging game that gives you plenty of interesting choices to make, doesn't overwhelm you with minutiae or confuse you with ambiguity, and cuts out all the boring fluff between interesting experiences.

We'll go into more details about the other aspects of our game later. I hope you've enjoyed this article, and I look forward to hearing and responding to your thoughtful comments, as always.

-Lars out