Not bad...
This is one of the best Defence games i've ever played. Okay, so the concept isn't too original, but you managed to pull it off. Still, I have a few suggestions:-
-Add a few more guns to the mix
-Make the damage and effects of each gun more realistic
-Put some really stupid guns in, for example a Gau-8 'avenger' or a cannon.
-Fix some of the bugs - on day 21 my minigun would not fire (even though it had been working perfectly for the last 2 days)
-Leave the rest as is
Ta!