I agree, a nice game, something that has been missing thus far! Apart from the graphics, it also needs some more polish (like an explanation what do the different towers do? What do the upgrades do and how much do they cost? Also switching between the towers via 1-4 on the keyboard doesn't seem to work properly for me. And then I'm playing this on a IIci and the screen being tucked to the upper-left corner. Any chance this could be made screen agnostic, e.g. to display centered no matter the resolution?
Good first version though, please keep at it! It's already fun to play in this form. It can become so much more. Thumbs up from me!