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Combining tower defense with a shooter - game design implications

The preconceptions you have about what the final product should look like often look great on paper, but do not fare so well in reality. We go on a journey through X-Morph: Defense development and look at the changes it went through over time.

X-Morph: Defense is a blend of tower defense and a frantic shoot'em up. It is an attempt to combine the best aspects of two seemingly opposite game genres – fast-paced action of a twin-stick shooter and the strategic depth of a tower defense game. Merging the base mechanics of these genres, without overburdening the player, required us to fight against a lot of common design clichés and our own misconceptions. This game design journey took us more than five years. Here’s how we did it.


Being a game developer has both its awesome and less-awesome sides. You can create basically anything and every idea can be viable, which is great. What isn't so great is the fact that if you let yourself loose you can come up with too much stuff. The story of X-Morph: Defense’s development is no different.

The core idea for the game was to combine the mechanics of a twin-stick shooter and tower defense. What it meant for us, was that we had to come up with two balanced game systems, both for the player-controlled ship and the AI-controlled defense towers. The challenge was to make the means of defense inseparable and complementary to each other. A few core ideas emerged, giving us a framework to build on.

The core concept of a tower defense game is that enemies attack along a known path and the player’s goal is to defend against them by placing towers along their route. One of our primary goals was to give the players ultimate freedom in where they can place their towers. Allowing the players to raise towers wherever they wanted meant that the enemy path could be obstructed and thus had to change. Without fixed attack routes we gave the player the choice to redirect the enemy forces. Given the open nature of our levels it would require a lot of towers to block passages. The idea of a laser fence was born - a tool introducing even more depth into the player’s decision-making process. 

Early mockup showing how the laser fence divides the battlefield.


The laser fence mechanics and freedom of placing your defenses introduced so much strategic depth you could lose yourself in this part of the game completely. It was further enhanced by introducing destructible environments. This topic is in fact so vast that we will have to come back to it in another article.

Placing laser fences during combat can be intense.


Specific tower designs turned out to be secondary to the laser fence mechanic. We wanted to make various kinds of towers, each specialized to deal with specific kinds of enemies. This was supposed to introduce variety and more strategic planning into the game, as we did not want our players to simply build a wall of ultimate supertowers and completely wreck the enemy. We had to make sure that all types of defensive structures had significant enough drawbacks.

While working on all of the above it was incredibly easy to stray away from the main idea behind the game – twin-stick tower defense. Since its inception X-Morph: Defense has been an action game. Had we included too many complex tower defense mechanics, the player would have had hardly any time to fly around and blow stuff up. On the other hand, if the players had got too few choices the game would not be fun at all. It soon turned out that lifting the burden of knowledge off of the player and streamlining our ideas was the most important decision we made.


At first our thought process was very simple – good tower defense games have a lot of enemies and towers to match against them. We obviously wanted our game to be good, so we came up with a large number of different tower types. Thus, the first collection of towers was born. What is interesting, though, is the fact that we introduced a couple of basic types and making the player choose from them. As such, a basic ground turret was introduced. The simplest design of them all, a tower shooting at ground units within range until there are no targets left. Since we wanted the fight to take place in the skies as well, the anti-air counterpart was born. Same stuff, different plane. Introducing complexity came at a cost we would have to pay later.

Early concept of various tower designs.


At this point in the development process we let our creativity completely loose. To get rid of heavier units, we made the armor piercing tower. Low fire rate made it nigh useless against infantry, however, the ability to fire continuously on one target proved to be a viable option against supertanks. But what if there were no tanks to blow up? We had a solution for that too – artillery, able to annihilate groups of small units over long distance. It also served as a means of protection, as it could destroy enemies endangering your carefully planned defensive layout.

Nothing fancy so far. How about a lightning generator? A tower so powerful it could destroy multiple flying units in one shot? Yup, got it. If we are going into using elements against the enemy, why not introduce a corrosive, damage-over-time kind of weapon? We made that too. But what about those fast units rushing my base? There you go buddy, a slowdown tower for you. And if you wanted to get extra fancy, you could place hidden portals, transporting enemies to a desired point of the map to meet their doom.

Early mockup showing the chain-lightning tower in action.


We set out to create something for every kind of adversary. Our playtesting showed, though, that giving the player 58 choices with 92 different kinds of add-ons was far too many. We had to gut the tower types, because of their redundancy. Let's take a look at the corrosive tower. You would want to use such a weapon on a heavy armoured enemy, to break through their armor. However, we already had an armor-piercing laser to do it. Introducing a second weapon to do essentially the same thing would be pointless, only resulting in players trying to figure out which one is better, whereas in reality both were great at doing the same thing.  It would become obvious that we had to drastically limit the number of ship weapons and towers.

These are all the tower types that remained in the game.


Given that at the beginning there were supposed to be time limits for setting up towers in X-Morph: Defense, there was simply not enough time to make a choice. This would discourage players and set the entry bar too high. We decided to remove the time limit for setting up towers in between enemy waves, since it only added unnecessary tension. Instead, we opted for intermissions between waves without any time limitations.

Through these decisions we managed to stick to our goals, while maintaining the delicate balance between the dual nature of our game. You could still experience intensive action, being overrun by various units from all sides of the map, yet there was a moment of peace after the storm. A calm break, where you could strategize to no end, placing towers where you wanted and setting up mazes for the little human insects. 


Before designing any tower we had to decide what parameters each of them would have. Cost was the first, as nothing comes for free, even if you are an intergalactic conqueror. Destructibility of towers meant they would have limited health. In order to give them some individuality they would come with different range, firepower, rate of fire, rotation angle, movement properties, influence range and a huge number of other parameters. 

This part of design received the same kind of treatment as tower types themselves. While some parameters became obsolete because of natural design shifts, others were removed on purpose. Take the influence area as an example. At first we wanted the alien structures to emit a metallic sort of infestation that other buildings could be placed on, much like the Zerg creep. Even though you would be able to raise new structures only within your influence range, they could be relocated freely later. However, it proved to limit the gameplay style too much, favoring the 'make a fortress' style of play. It also introduced another layer of micromanagement, which wouldn't go well with our action-based gameplay. The influence area made it to the final version of the game, but just as a visual gimmick.

Influence area expands around all X-Morph structures. It can change trees into lifeless metallic husks.


Building costs also received a major overhaul. Most strategy games will present you with some kind of money or other resources. In order to build a basic tower you need, for example, 100 gold. Artillery is more expensive, at, let's say, 175 gold. X-Morph had a similar system, with every structure coming with an associated cost, specific for its kind. As a result you would have to check the costs time and again in order to see if you have enough money to build what you want. The solution to this was unification of costs.

A single tower can morph into any other tower type. It makes modifying tower setups much easier.


Resources in our game amount to energy bars. When you gather one energy bar, you can build one basic tower. It can be further morphed into an improved tower for an additional energy bar, giving it new functionality and a more specific function. It creates a much simpler model, that needs very little introduction and is easily understandable even for new players. Another consequence is that you can set up a maze first, and worry about changing the types later, as the base structure is already where you want it to be.

Another feature that changed during the course of development was tower relocation. Originally we wanted our towers to crawl and take time to move from one spot to another. Instead we opted for instant transfer. Therefore, if you make a mistake or overlook something you can easily fix it in the heat of battle. It doesn't cost you anything, neither does getting rid of a tower completely. It is a natural consequence of simplifying costs. We didn't want anyone to deal with fractions in game while there are thousands of enemies to shoot.

Moving towers is free and almost instant.


It is sometimes hard to let go of some concepts, especially if you already spent weeks making the model, adjusting parameters, polishing the balance. However, the game is for the players, and even though it is your creative outlet sometimes you have to limit yourself not to overburden your audience. It was a hard, but necessary lesson to learn for us.


Art design was very important to our gameplay goals and it was primarily driven by function. Our graphics team went through several iterations of tower design. At the very beginning we wanted the style of alien structures to resemble machine-flesh. We also briefly experimented with crystalline structures, and Giger-like monster towers.

Early mockups suggested a machine-flesh approach with a H.R. Giger vibe.


To be honest, machine-flesh presented another problem. All those awesome details would not be visible. Moreover, at the zoom level of our camera, they would simply form a chaotic mess of pixels, rendering our goal of achieving visual clarity virtually impossible to reach. Moving the camera closer was out of option as well, since it would limit vision and obstruct the strategic layer of the game. A much more sleek design was necessary.

We finally settled on metallic constructs with beams of energy decorating them and emphasizing their purpose.

Hard, metal structures with energetic highlights worked best in contrast with world scenery.


The art style that we decided on was also a consequence of following the lore that we created together with our design goals. In order to achieve consistency and solid design we had to discard many ideas which we really loved. Of course, nothing goes to waste and it is possible that one day we will use those designs, should the stars align. As you can see, we had to abandon a lot of ideas. Sometimes it is not difficult at all, you just invent something better and go with it. The problem arises when you have worked with something for a long time, really determined to make it work.


All things considered, the development of X-Morph: Defense was a valuable lesson in modesty. We learnt that less is more most of the time, and that nothing can surpass clarity and simplicity. There were more changes along the way in other aspects of the game’s design, but they are also long stories, worth multiple articles each. You can stay tuned for more posts like this, we plan to publish more regularly. In the meantime, you can follow us on Twitter and Facebook, and perhaps give the game a shot! We promise it's worth it! :)




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