A guard tower
Anti Infantry Tower
High-velocity machine gun
It is essentially a sandbagged observation post on an elevated wooden platform, outfitted with a powerful machine gun, excellent against invading infantry. Guard Towers provided a manned defense to counter hostile ground attacks.
In Tiberian Dawn Guard Towers require 500 credits to construct and use machinegun fire to deal with infantry effectively. They are weak against more heavily armored units and impotent to air units. If managed properly Rocket infantry and Nod Light Tanks can fire at the structure from outside its shooting range.
The Guard Tower does not use much power and can still function without it. They also have the ability and purpose of detecting Nod Stealth Tanks when they are near. In one game cut-scene a Guard Tower is seen observing a retreating Nod assault party, only to be vaporized later by a (presumably commandeered) Ion Cannon.
The Guard Tower's machine gun's high rate of fire and noise is similar to the German Army's MG3 machine gun, which is a modernized version of Nazi Germany's MG42 machine gun, which was the replacement of the MG34 machine gun.