I agree with smokehouse, retrogarden, and carlcarlson. If I were able to demo WiiWare games for a minute or two, I probably would've bought some that I'm on the fence about.
Yes, some WiiWare games are horrible (SPOGS Racing, Pong Toss: Frat Party Games, Protothea, etc.), but some are great and look very nice (LostWinds, Final Fantasy Crystal Chronicles: My Life as a King, Dr. Mario Online Rx, etc.). Overall, Defend Your Castle received a 6.7 out of 10. It's mindless fun, but not the best example of what can be done on WiiWare.
Next time, check the review scores before you buy:
Top 30 and Bottom 40 Wii games (including WiiWare)