It's a good start. 3d makes it pretty difficult, since there's no visual indication of how much range a tower has, and it's easy to end up with a blind spot where no tower can reach the aliens hitting your base. Letting bullets phase through the base would solve that, or shoving a buildable platform closer on the side that isn't as protected.
Game doesn't end once the tower blows up, it keeps going to the next wave, but with the aliens just hanging out at their spawn.
good work!