Amazing game.  I haven't had a lot of time to play it, but I did finally manage to beat level 45.  (Not in the way I'd like though, I just bought extra lives to survive the last two levels.)  I suppose that would have been my fourth or so try (I only got to level 30 something before I knew turrets were upgradeable, then bought too many depots too quickly, and the third time I died to wave 43).  As such, there are only a couple of suggestions that I have.
First of all, why is it necessary to ask for the desired number of waves?  Wouldn't it be easier to assume 45 and award bronze and silver metals when the player completes 10 or 24 waves?
Secondly, an unlimited wave mode would be really, really nice, but I see a couple of difficulties.  First of all, there is a maximum obtainable DPS on any map (probably a mixture of fire turrets and sentinel outposts that covers every buildable square on the map).  Second, turret types become obsoleted in later levels (Earth, Ice, and the two Barricades IMHO), thus reducing the need for strategy (ok, I have tons of money, what's my highest DPS turret?).  One idea would be to expand the map area (perhaps rotate the map and connect its beginning to the original's exit).  Another would be to give the cheaper turrets special abilities when upgraded (1,2,3,4,5 second stop to mix-up the enemy order, ice might weaken the enemy to fire, etc.).  A third idea would be to have special unlimited wave technology, such as Earth Turret: 1.5X damage, +1 target, which could be bought repetitively (but more expensively each time).