This article is non-canon.
The subject matter of this article does not take place in the "real" Half-Life universe.
The Wall Turret is a Combine turret that is built into the wall of certain rooms in Combine buildings or structures. These turrets were to be very unaffected by and have a similar range as the Combine Ceiling Turret. Just like the Ceiling Turret they cannot be removed from their walls.
The turrets, however, were cut during the 2003 era of Half-Life 2 and its model can still be found in the Half-Life 2 Beta files.
It did however appear in one known map known as proto_c17combine where it was hanging on a wall with some old posters right by a Combine Smart Barrier on the right. The turret, however, does not feature any animations or AI rather its just a model. It's rumored that it was to behave similarly to the Ceiling Turret.