The turret is a building that cost $65. It has a cylinder shaped column for the base and the top resembles a tesla coil which shoots rays at enemy units. It does count as a building, contrary to other belief. However the turret (As of The Conquerors 2 and in the current iteration), does not create an area where you can build off of.
This is mainly used for defending bases from enemy troops or from defending power plants from early game light soldier rushes. It is good against a couple of enemy troops but can easily be swarmed and killed.
They can be built very quickly and you can with great speed prop up 3 turrets and 2 anti-air turrets on some offensive point, say, a recently captured enemy energy location.
Battleships, destroyers, and aircraft carriers are capable of outranging turrets fairly easily, so building these near naval yards for protection should be done carefully, to make sure that the investment in them is not wasted. Heavy tanks and especially Artillery also outrange turrets, though it is kind of hard to get the tank right at the edge of the turret's range.