DtDTD is a fairly simple game; your objective is to keep continuous waves of enemies from reaching the end of the level. You do this by attacking them with Towers.
You will find that there are over 100 towers, motion sensitive bombs, tanks, and planes! You will strategize to use these different things to keep the enemy from reaching the end of the level.
The game has no end, so you can keep playing for as long as you like, but if you do get bored: there are two different game modes, with three difficulty levels each.