The Sentry gun posts, also known as Cannon towers, were a type of defensive emplacement used by the Goa'uld.
These large tower based weapons were capable of being hidden underground and sprang forth as obelisks with a single operator. This individual sat at the top of the machine and once inside, they were well protected from personnel weapons. A single Staff cannon resides right above the weapon seat. The operator made use of a sensor eye to aid them in targeting enemies with two controls which worked as levers in the movement of the main weapon.
The sentry gun posts had a form of computerized aid for the user as the oval shaped device was present by the left eye. Target data was present for the weapon operator and a target reticle was capable of being displayed which enhanced the aim of the sentry. Furthermore, they allowed for an increased capacity to aim in the dark.
While potent static weapon defenses, these gun posts were capable of being destroyed by fighter weapons.
The Goa'uld queen Hathor established at least two sentry gun posts on one of her planets. After SG-1 discovered her deception in believing that they had been frozen and were in the future, they attempted to escape when the sentry posts emerged from the ground and began pinning the humans in their place preventing their escape.
Colonel Jack O'Neill was targeted by one of the sentry posts but it was destroyed by a Needle Threader which was piloted by Major General George S. Hammond and Free Jaffa Teal'c Bra'tac. (SG1: "Into the Fire")