Design Process - How to select a 'Tile'?

During the development of the first prototype, I found out defining [how to select "tile"] is a hard thing since it is related to the view of the camera, the shapes of traps & towers, how many traps/towers can be built on one rock or any other tiles.

Some Constraints & Reference:

3D (different from Dungeon Warfare's 2D)

Top-Down View (different from Orcs Must Die's TPS View)