Complex sentence: It was armored and reinforced to support the weight of the mount, and a person standing in the upper handling room could look up and see the bottom of the gun mount inside the training circle on which the mount rotated.
Split into simpler sentences: It was armored and reinforced to support the weight of the mount. A person standing in the upper handling room could look up and see the bottom of the gun mount inside the training circle on which the mount rotated.