Yeah, Tower Madness is truly an unsung TD when people are asked to list their favorites, but it really deserves more recognition. I really enjoy it, it's challenging, the graphics are fun, and it has awesome online features no other TD has. The downloadable maps and ability to view replays of the tops scores is absolutely great.
Still trying to beat the Insane level in Tower Madness. But I did get on the leaderboard for the WindingWoods (currently #30) map.