I have just spent the past long weekend on a coding binge, 20+ hours of lovely Ruby. The end result is a tower defence "game" with easy to use base classes, making it easy for anyone to add a level, a tower or even a new bullet type. The "game" is in a playable state, and anyone who his familiar with rubygame should check it out. The "game" is available via GIT.
The following is a screenshot of ruby-tower-defence in action: