Hi! We are glad that you like our game.
A creep is a number of textures that replace each other on a ground plane like frames. We also have something like a “build-up map” which is a low resolution grayscale texture where black pixels are free to build up and white pixels are not. It is generated at runtime from the creep textures and the position and size of towers in the game logic, and is used to check when the player wants to build something. This could be implemented in more lightweight way by using a bool[,] map instead of a texture, but Unity has some useful functions for dealing with textures, so we'll use them to speed things up.