I was introduced to Sol Survivor by someone who'd read my review on Heretic and asked if I'd ever played it using the Doomsday engine. He had an extra copy that he gifted me after we spent a couple hours kicking each other's asses in some Dukematches.
This is my first "real" Tower-Defense game, and it's a great way to be introduced to the genre. Before Sol Survivor, my only glimpse into this genre were the UMS "Turret Defense" and "Pylon Defense" user-created maps for Battle.net
StarCraft matches (classic, mind you, not II).
This is an old game by today's standards, but I would consider it and oldie-but-goodie. However, the multiplayer pool is quite small, so only buy if you intend to play the single-player or have friends who want to play with you. It's easy to get into, and the music is kick-♥♥♥. The graphics are also quite contemporary, and I'm rather impressed that a tower-defense game has that much love put into it. Story is somewhat nonexistent, but it's a moot point when you're watching your towers launch volley after volley of defensive flak to a kickin' soundtrack. If you buy this game based on my review, feel free to look me up! I'd love to enter a match with you.