Sunken Defense is a custom map for Blizzard Entertainment's 1998 real-time strategy video game StarCraft.

Custom maps can be created using the packaged-with-game utility StarEdit, and typically place players in a new and unusual gameplay situations. The origin of Sunken Defense was a map called Tower Defense, in which the player was forced to fight off waves of enemy attackers using no (or very few) soldiers, only extensive networks of defensive 'towers.' In the case of Sunken Defense, the player is restricted to only the Zerg Sunken Colony, which uses a single giant tentacle to attack nearby hostiles. This structure gives the map its name.

Each player finds his- or herself halfway down an aisle, with only enough resources to build a few Sunken Colonies. Computer-controlled enemy forces then begin to meander down the aisle. The objective is to prevent all of them from reaching the other side, and if any player should allow but one enemy through, the game is over and everybody loses. The Sunken Colonies are, obviously, the main means by which players deliver themselves from instant death. Besides killing the meek and rather oblivious enemy hordes (with few exceptions, none use their weapons), they can be fashioned into various mazes and walled paths. If the enemy finds their way completely blocked, however, they will begin to attack the Sunkens indiscrimately; even at lower difficulty levels, these onslaughts can easily disembowel a player's formation, and they must hurry to rebuild. Once the current wave has cleared, the next begins, generally to a total of thirty.

Players receive more resources depending on how many enemies they kill (some maps use cross patterns or place two players on a single long aisle, thus giving their allies a chance to save the game by picking up the enemies--and the resources they represent--that the first player missed). These resources can be spent on more Sunken Colonies, to add to one's standing defense, or on special Heroes, actual soldiers who can move around and generally do more damage than existing Sunkens. They are the second key to survival: the Sunken formations exist merely to contain the enemy, while the player's Heroes do most of the actual slaughtering--enemies from higher waves will sometimes be practically impervious to Sunken Colony attacks. Furthermore, hordes will sometimes get impatient and begin to attack the player's Sunken formation even if they are not totally walled in. The objective is not just to kill them, but kill them relatively quickly, and Heroes are the only way to do this. However, in some versions of Sunken Defense, heroes have enough armor to sustain damage from enemy units and some players use them to make walls. In fact, in some cases players can abuse the AI and the hordes don't even retaliate when a wall is in place.

Sunken Defense maps typically support six players. A series of side-by-side aisles was probably the original configuration, but later maps contain players in a two-across-by-three-down arrangement, and others use two long aisles bisected by a third shorter one, each supporting two players and with enemy hordes crossing each other in the middle.