a good idea for a tower defense game, with 'expanding' doing well by adding things around the core, but there is limits.
that being said, I can see why you put limits, because this game is only fun in the short term, for about less than 20 waves, the enemies wasn't much of an issue once I managed to get projectiles in place, and adding more and more, I didn't have to do much, the only enemy that was a problem was the white wizard, but then afterwards they wasn't an issue, eventually, I upgraded so much that I couldn't see the enemies anymore, they were taken out before they could appear on the screen.
and to give you an idea, I've actually played this game up to wave 100, by then, all of the spaces were used an upgraded, and if I do try to fire, it begin to lag.
here's some picture evidence
so yeah, good for a short game, but if you are aiming for long term gaming, I suggest for tougher enemies or make them stronger as the wave goes on.