Defend the Keep, or DTK for short is a PW "gamemode". It features one or more defending teams who are tasked with surviving wave after wave of enemy units, the match usually ends when the defenders are wiped out, a certain wave has been reached, or not at all of course.
In recent times two variations on the DTK gamemode have come about, these are "Classic" and "Premade" there is also a relatively unknown version in which players are more often then not given only one unit.
Classic DTK is when the defenders are given a limited amount of units and must create more as the game goes on, through use of their fortification's machinery or the machinery they have built up during the game. Many old timers prefer this mode as it keeps the feeling of desperation up and provides a more interesting match enabling you to use more environmental tactics (as without them, you'd likely lose)
Premade DTK is when the defenders are given a large amount of units, and are almost always located in a fortification as a result of these waves usually stay consistent in their size, more often then not however, the defenders are unable to make more units.
Build to Survive Edit
This DTK is when the defenders are never given a fortification, and must instead build up their own in several "free" turns, usually this gamemode has a large number of defenders with a single unit of "Hero" type power, they must then band together to create a fortification, or survive out in the open against waves which usually come from all directions.