The pinnacle of Tower Defense; Defense Grid nails the genre. The presentation value is high with sleek visuals and a well-crafted narrator that guides you through the game's universe, but what really matters is gameplay, and Defense Grid nails it in the campaign and the many, many diverse modes it packs along. You can challenge yourself with limited resources or engage in a zen-like grinder session as you slowly roast your way to nigh unlimited riches.

By far the best money I've ever spent on Steam, I've also purchased all the DLC and already prepurchased the sequel via kickstarter. If you like strategy in general and TD especially, you'll want all the DLC, but definitely don't miss You Monster, because the only thing this game was really missing in the first run was GLaDOS. She compliments the game's existing narration nicely.