The Sentinel and the Gaat Gun, Arm and Core's Level 2 heavy laser towers, are an important part of a developing base. They can provide an invaluable defense, where rocket towers are too weak, and plasma turrets are too inaccurate, these sum it up nicely. They require a store of energy to fire, so make sure that you still have some other land defense.
The Arm Sentinel
The Sentinel is the Arm HLT.
The Core Gaat Gun
The Core version, the Gaat Gun is a powerful triple barreled laser tower.