(originally posted 1st June 2011)
From the start I wanted a tower defense game in 3D. Not just one which is rendered in 3D, like Defense Grid, or an isometric view like Doomsday Defense, but one where the gameplay takes place in three dimensions.
Most tower defense games are linear, with enemies travelling along paths to an exit or goal. These games can be very well done, but the gameplay always comes down to overpowering a parade of enemies with upgrades, as any strategy without upgrades is quickly discovered.
A few tower defense games are pathing, which means the paths enemies take can be blocked. Towers are used both to shoot enemies and to block their route, making them take a different route. These are relatively uncommon, perhaps as they require at least a little bit of AI/pathfinding code. But the gameplay is still very predictable: create a long path for the enemies and place your best towers where they cover the most of the path. Some have flying or other enemies not stopped by walls but that doesn’t much change the strategy: there may be a special gun that takes them out, so a fraction of resources need to be spent to it.
I wanted to make a game that was truly 3D, or at least multi-layered (as Flash doesn’t do 3D very well), so enemies and strategy were not confined to the plane. My first idea for this was to have different layers above ground: with crawling enemies which were blocked by everything but flyers that were blocked only by tall towers. The flyers could fly over turrets but might take damage doing so. They might also drop things: crawling enemies behind tower walls.
To allow creeps to get around the walls of towers I introduced tunnels: this would make it possible to have two mazes, one at tower level above the turrets, one lower down between the towers and turrets and underground. But I soon decided that was too complex. Asking users to construct mazes on two levels at once would put most users off. It would also be very difficult to represent onscreen.
So the flyers were dropped before I really started. The only remnant of them is in the two sizes of gun, with the taller tower able to shoot further because of its height. The tunnels though I kept. On their own they added an extra layer, below ground, introducing just enough complexity but not too much.