With multiple targets present, a single Sunken Colony can be manually targeted using the Attack (or right-click) command. An example of this manual targeting is in Terran match-ups to kill the attacking Firebats first while other Zerg units such as the Zerglings can attack the Marines.
A common misconception of the Sunken Colony is that it is a melee attack due to its animation, but it is not effective under Dark Swarm
The morphing from Creep to Sunken Colony reduces its hit points by 100. As such, when an attack takes place while the Sunken Colony is undergoing the morphing process, it is advantageous to wait until morphing completes, as it can sustain less HP than the Creep Colony.
The ability of the Sunken Colony to kill Marines (without armor upgrades) in two hits helps defend Zerg bases in the early and mid game. They are particularly robust when constructed in groups. Further, the Sunken Colony has farther range than a Bunker of Marines. Their effectiveness in the late game is reduced due to the Seige Tank, but Sunken Colonies are particularly useful in countering bio drops, alongside Scourge.
Because Protoss units such as the Zealot can sustain a lot of hits from the Sunken Colonies, the Sunkens are often used in large numbers to provide adequate defense. Similar to Firebats being manually targeted to increase the Sunken Colonies' life span during a Firebat/Marine/Medic attack, Archons can be manually targeted when facing a mixed army such as that of Zealots and Archons. Proper positioning of the Sunken Colony is an important part of the Zerg Sim City defense. Additionally, if a Zerg is Photon Cannon rushed, Sunken Colonies are particularly effective because of their greater attack damage and higher hit points compared to the Cannons.
Sunken Colonies are used to help defend the Zerg that is Teching to Mutalisks faster or going for an early expansion when facing a Zergling attack. Sometimes offensive Sunken Colonies are used in this match-up when one player morphs a Sunken Colony on the opponent’s Creep. In this case, it is placed out of range of the opponent's Sunken Colony.