On the level with the couple looking at the sunrise, I keep clicking on a building, and it flashes, but it doesn't register as a difference. It was the last one, and I got it with the Hint key. You might want to include a "restart level" function so that I can actually finish this without redoing the whole game.
Weapon Powerups are really, really far apart. Makes it much harder than it needs to be, and really detracts from the parody, as that's not the case with most defense games.