Defense Grid 2 is the follow up to the original Defense Grid. Originally slated to be a kickstarter game alongside DLC for the original game, the kickstarter didn't meet the necessary goal to get DG2 funded. The kickstarter funds were however later augmented with some traditional funding to complete the game.
While closely following the formula of the original, it's built from the ground up on an entirely new engine, allowing for dynamically changing maps during levels, multiplayer support, and improved leaderboards. It also adds a variety of new tower types, including the "boost" tower, which can be used as a low-cost aid to shaping, or to elevate the position and increase the damage output of another tower placed on top of it.