Structure Info
Sunken Colonies are one of the two defensive building created by the Zerg Swarm, These colonies shoot a tentacle underground to impale any enemy ground unit as it emerges back to ground level. This does a decently large amount of damage.
 Game Usage
 Vs. Terran
Sunkens are used to defend against Marines early, as they fall in 2 hits.
 Vs. Zerg
When facing another Zerg, Sunkens can be made to defend against greater amounts of Zerglings.
 Vs. Protoss
When walled off with a Hatchery and another building, Sunkens can effectively defend against Zealot aggression. In the later stages, a single Sunken is usually placed with mining Drones to defend expansions from Dark Templar harassment.