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Don't drink the Coolant

As the spiritual successor to Grail Guards this cooperative hero defense game puts you into the role of two engineers who have to keep a bunch of greedy slurperslugs from drinking the power core’s coolant liquid.




The two players start their game in the middle of the map, each on their respective side of the walkway. Once the alarm sounds, the enemy sluperslugs will start to spawn from the four lanes to the left and right of the map and slither towards the coolant tank in the upper middle of the map. Each new enemy wave is announced by an alarm and will become more challenging than the previous one, so both players will have to help each other out by supporting their friend in combat and restoring the power core’s health with items.  Once the battery bar on top of the power core is fully charged, the players will be victorious. However, if the power core gets destroyed before that, the greedy sluperslugs will cause a city-wide power blackout which results in the players getting fired.


Design Notes - the Predecessor

Ever since we presented our game Grail Guards in the game jam I had wanted to create some kind of spiritual successor where we could implement all the ideas that we had to cut due to the game jam’s time constraints. And with the game from the game jam, we already had some kind of tried and tested prototype for the core gameplay that we could build upon. All of these factors led to me deciding to pitch the game concept for our first project and after some in-group discussion, we decided to merge my idea for core gameplay with the futuristic cyberpunk setting from my co-designer’s pitch. And thus, Don’t drink the Coolant began to take shape...



What I found most interesting is how Don’t drink the Coolant is a prime example of how focussing on a specific aspect can completely change the whole game and even though its hero defense core gameplay was based on the pre-existing formula from Grail Guards I would still say each of them can stand on its own as a game. Grail Guards as a base had many features that we could expand upon: there was the idea of giving the players different power-ups or maybe even different but complementary combat skills for each player to add more variety. Furthermore, one idea that we had discussed during the game jam but had to cut was introducing different enemies with individual behavior patterns. Last but not least, the map for Grail Guards was an empty plane with the grail in the middle - once again an aspect that we could change to give Don’t drink the Coolant its individual style.

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Instead of damaging the enemy automatically on contact, the players now have to do a timed button input to perform a 3-chain melee attack. In the later stages, we expanded this combo to include a finishing blow with double damage if the player pressed a different button for the third attack. Another small change that made a big difference was the addition of a sprint function that gave the player a speed-boost, thereby allowing them to cross the bridge between the lanes faster or preemptively run into approaching enemies.

Design Notes - expanding the Combat

Initially, the coop aspect of the game was supposed to be asymmetrical with one player only using ranged attacks while the other player had a melee weapon. This had to be cut in the early development phase of the project and we decided to give both players the same skillset and therefore more freedom to divide their roles for themselves.

Since our supervisors pointed out that spamming the attack button might be a potential danger that could make the gameplay repetitive, we decided to add the finishing combo as a reward for players who refrained from button-mashing.



This field has become the biggest expansion compared to our original idea. While we had to cut a ranged variant that would have shot projectiles we were able to create three distinct enemy types by simply scaling the values for speed, damage, and health. And even though this idea is hardly a new one (after all a strong and a fast enemy variant are the most common version in many games) I still liked how such simple tweaking not only changed the gameplay but also required the players to apply different tactics against each enemy type.

See all enemy types



The pink slurperslugs with the polka dot pattern are our standard enemies and possess medium stats for speed, health, and damage. Since they are so average in each regard the players are free to decide if they would team up dispose of the slugs faster or just whack them each on their own.



Those little ones with the arrow pattern are the fast but weak variants. With their high speed stats, they can move faster than the player and reach the coolant tank within seconds. Once they arrived at the tank they would switch to an attack pattern that did low damage but had a very rapid attack frequency, so despite their low strength a swarm of Sprinters can become dangerous if left unattended for too long. Luckily, those variants only take one hit to be killed, and due to their small size, a player can take out many of those enemies with only one strike.



With its huge size and the spiky design, the Moloch looks imposing and that for good reason: not only can they do massive damage to the tank with a single hit, they also have a large pool of health that takes its time until it is depleted.


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Design Notes - designing new Enemy Types

A welcome side-effect of changing the enemies’ values to make them more distinct was that it simultaneously created a tactic to take them out most efficiently. While the regular pink Grunt didn’t require any special behavior, the other two slugs did:

The small and fast Sprinters, for example, were so fast that they would outrun (outslither?) the player with ease - but die in one hit. So instead of running after them, the player could wait until all of them were bunched up at the coolant tank and take them out in one single blow. The Moloch, on the other hand, was designed to enforce more cooperation between the two players. With its huge health pool, it could take about 25 hits, so teaming up against a Moloch was the fastest way to get rid of it.

Additionally to those different variants, Don’t drink the Coolant also introduced a wave system with a specific combination of enemies compared to Grail Guards’ randomized spawning. Once again, this modification was intended to strengthen the game’s coop aspect, in this case, to add a bit more tactic to it. The waves were specifically designed to direct the player between the two lanes: a major onslaught of numerous enemies or had them spawn in high frequency was usually focussed on one side of the level and designed to make both players join forces to defend this side. Whereas a smaller, more balanced amount of enemies on both sides of the level map was intended to split up the two players so everybody could defend their own side.

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Design Notes - designing the Waves

Overall, the enemies spawn in nine waves that are segmented into three phases each:

  • Phase 1 serves as a form of tutorial, has low spawning numbers and introduces the three different enemy types.

  • Phase 2 has the enemies spawn from different areas in a way that forces the players to move between their lanes, team up on one side, or split up to defend both of the lanes.

  • Phase 3 builds on the spawning pattern introduced in Phase 2 but cranks up the number of enemies to make the last minutes of the game more challenging.


In order to further encourage player coop but also make the game more challenging we decided to create a map topography that divided the area into two symmetrical segments. Each segment possessed its own coolant tank (the object that players had to defend) with a shared health-pool - but the two tanks were positioned on different vertical lanes and in order to reach the other tank a player had to traverse a bridge that connected the two parts. Furthermore, the enemies no longer spawned from random positions outside of the map.

See new map design

Instead, there were four horizontal lanes  (two on each side) from which the enemies would spawn. Said enemy lanes were of different length with the upper ones leading directly to the coolant tank while the lower lanes were bent, meaning that enemies coming from this lane would take much longer until they reached to tanks. This created an implied but distinct priority for which lane the player had to focus on once the waves started spawning.


Design Notes - old Ideas for the Map

The map has undergone quite a number of reiterations until its current form. We experimented with a design that had a more centralized position for the power core and multiple lanes formed like a spiderweb but decided against it because it was to easy to defend.

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Another reiteration that featured separated coolant tanks and a larger map had to be scrapped due to difficulties with the camera (that had to take two players into consideration after all).

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As a person who has been part of both development processes and tested both of the two games, it made me glad to observe that each one plays differently and is still able to stand on its own, even if they use the same core gameplay.

Due to its open map, and centralized point of defense Grail Guards has a much faster pace than Don’t drink the Coolant and can even be hectic at times. The fact that enemies can spawn from everywhere outside of the map and have an increasing number and spawning rate the player is more vulnerable and the difficulty curve slower but rising higher during the end when the player is literally bombarded with skeletons. Furthermore, the healing item has a stronger risk-reward element to it since the player can collect two of them but is unable to attack while holding the items, something we cut from the other game.

Don’t drink the Coolant had much more time for reiteration and balancing and it shows both in terms of difficulty curve (steadier and not as steep) and the cooperative aspect. Overall, I would say that this game demands more tactical planning because of the divided level map and the different enemy variants. And since there is a short buffer time between each wave the players have more time to communicate and arrange their tactic as well.


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