During Steam's recent Summer Camp promotion, I was able to score the expansion packs for Defense Grid for free, so now I'm back playing that game
way too much. Last year I played all the way through Gemcraft: Chapter 1 (
http://armorgames.com/play/1716/gemcraft) and played through a good chunk of Chapter 0. (I've never spent so much time playing a free Flash game) I love this style of gameplay.
I know that Rampart is considered the grandfather of this genre, but were there any other tower defense games released for 8/16-bit systems?