Tower defence games often follow a formula, whereby you obey certain rules and fulfill certain criteria. Good tower defence games make this process happen invisibly and provide an experience that is both addictive and challenging, games like the seminal Defence Grid, which is considered the benchmark upon which all other Tower Defence games must be looked upon.
Comparing Terrorhedron to a Tower Defence game isn't really possible, because if Tower Defence is a formula, then Terrorhedron is algebra. It is mathematical purity and brilliance, expressed in abstract form. In most Tower Defence games you have a fixed path upon which the creeps will run and try to get to your base, you may be able to mess with this path by placing towers down, or you may have fixed locations upon which to build your towers.
Technically both apply here, but where it departs from the norm in this case is that the tower build points are merely bases, and from there you add lego like snap connectors from which towers can grow, tree like from the root node. Simple turrets become crazy branch and leaf arrangements with dozens of turrets hanging off of them, all pointing in different directions, programmed with different priorities and having different lines of fire, and in this game, LOS is fully obeyed, so you have to pay attention such that you don't obstruct your own towers.
The game takes full advantage of this, with levels working on every single plane of a three dimensional maze, often weaving complicated, tangled paths over cubes or more complex surfaces, allowing you to maximise your opportunities if you're clever and are willing to be imaginative, and that's the key bit, because you can use the snap connectors freely, the only limitation is really funds and how crazy you can get with the tower layout, want to build mighty archways with laser turrets hanging off? Go nuts. I just did for a section of the first level, and it worked beautifully. Here, you -build- the towers, step by step.
With such an expansive canvas, the aesthetic comes as a relief, being clean and abstract, minimalist but without looking amateur, game looks slick, well rendered, and has a thumping soundtrack, and doesn't suffer any framerate issues, regardless of what mayhem is going on at the time, and the clean design of the level layout allows you to keep focussed on the action. Doing the tutorial is mandatory this time, because Terrorhedron requires you to adapt to the fact that this game will have you building corners, arches, bridges, and all kinds of things to create your own defensive setups.
The only minor complaint is the fact that towers are gated behind some sort of "experience" wall, so intially you don't really get the full Terrorhedron feel, at least until you've unlocked the snap towers, but once you have, the game opens up dramatically, and then the levels become your toybox.
Tower defence is maths, Terrorhedron is the degree course.