The support towers were large elevator shafts supporting the Haven Palace during Jak II. Towering over the city, these large bowed constructions were connected to the Palace via heavily defended support cables that helped to stabilize the Palace's top-heavy design. Jak once rode an elevator to the top of a support tower and traversed the support cable in order to infiltrate the Palace in the mission "Ride elevator up to palace". Aside from this mission, the support towers are not prominently featured throughout the game, but can be seen from the ground towering over the landscape.
The support beams themselves are guarded by sweeper turrets, sentry guns, rotating electrified blades, spiked and electrified platforms, and trap doors, used to deter people from trying to infiltrate the Palace. Other attempts by the Baron to protect the Palace was disabling the support tower elevators, which Vin was able to reactivate by routing some B-Zone conduit lines through a bypass grid to provide electricity to the number five capacitor array, connected to all five towers (per request from Jak).
When the Palace was bombed in the beginning of the War for Haven City, the support towers collapsed with it. The support beams consequentially fell on and destroyed many buildings across the city, most notably in the Port and the Industrial Section. Some support beams blocked access to sectors like the eastern Bazaar causing its abandonment.