Would really like to see a finished version
Seems like a really nice little demo... Well, the idea is good and the coding seems nice. I'd like it more if all the buildings were usable... Maybe even a scroll so that the levels can be bigger. Wouldn't be to hard to do since you've already got the base of the coding done already. Just make the buildings into "Objects" and store all the locations in a 2d array. Not sure how this would effect the enemy AI, but it would be interesting. It would also be really fancy if it was projected as a pseudo 3d map... also not too hard at all, but maybe a bit too much (but then the graphics could remain simple but give the look of alot more effort).
Maybe add a little menu system so you can earn add on abbilities to apply to structures to strengthen the defense or something.
I really hope there is a finished version, looks like it could be fun.