Never have I worked on a project so full of mechanical moving parts and gears and guns and turrets and missiles and rockets... You get the idea ;)
There is a very industrial feel to the game, the theme made it very easy to start with base assets, and then refine them down into smaller more punchy sounds (in order to help combat a problem I feel a lot of tower defense games are faced with: Hectic audio due to the sheer number of towers and enemies on screen in later waves. Another method we used was limiting sounds per bus, assigning certain sounds or groups of sounds to specific buses. Additionally we had the programmers do a radius check around the player to help prioritize which towers/enemies to play in situations where things could get severely overloaded)
On a totally related random note: The score ticker I created for this was from a plastic breaking session done a few years prior. While breaking jewel cases I'd gotten quite a few interesting performances where the plastic bent far more than I anticipated and snapped much sooner than I'd thought after bending. The result is essentially what you hear for that ticker.
Another thing I really enjoyed about this game was working with Dan Gooding, a fellow game audio engineer and incredibly skilled composer. The idea we had for this game was dynamic music that changed for each new wave (the story mode of the game was set to 10 waves so we used that as the basis for the system). In order to keep memory usage low, I'd set up the system to only ever play at most 2 tracks at a time (during transitions from one piece to another). Each layer was bounced with it's corresponding parts, as opposed to simultaneously playing many tracks and muting/un-muting them as needed.