Awesome Game :-)
All I can say is 'more' lol - I loved the guns (pistol, high-power pistol, uzi, and machine gun), the foes (gray large stationary target, red medium stationary target, green small stationary target, gray moving target, and the turrets, both aiming and non), and the cut-scenes.
For players: don't be shy about using your ammo, it (like your life) gets replenished after each round (and steadily increases, as well! but ammo doesn't carry over round to round)
Here are the level codes (note that they ARE case-sensitive):
01 1JTG4N
02 2FT4MF
03 3dEHBD
04 4JF18S
05 5MV6RG
06 6IH84H
07 7F6JAQ
08 8TH7KJ
09 9FU74V
10 10DFH3
11 11H87G
12 12EVUT
13 13J95G
14 14B8EG
15 15DFHG
16 16J873
17 178js6
18 188HJE
19 19GHEU
I'm very eagerly looking forward to any continuations you make of this, and would only suggest more levels, guns, enemies, etc. (consider a laser that bounces off wals / goes through targets, stuff like that)
:-)