Gamer Defense is a tower defense game featuring characters from popular video game franchises and includes retro minigames. You can play it on Greenfoot's website.
I was part of a group of five students that created this game using Greenfoot. We created it for an intro to computer science class. Developing the game was fun, although we didn't really know what we were doing.
The game is basically a tower defense game with characters "borrowed" from other popular video games. But there's a few things that set our game apart. For example, we added unique towers and enemies. Link performs his signature spin attack, Pac-Man goes backward on a lane, eating all enemies in his path, and Bob-ombs blow up nearby towers.
My favorite addition, however, is the retro minigames. Sometimes enemies will drop power-ups, which, when clicked, activate a minigame such as Pong or Space Invaders. These minigames allow the player to temporarily kill on-screen enemies with a different type of gameplay.
In my opinion, Gamer Defense turned out very well. I found it fun to play, and sometimes didn't work on developing it because I was distracted by playing it.