After defeating the horseman, follow the wide passageway to the south of the main room.
Here you'll find a raised gate and a pulley mechanism.
Stand on the step below the gate, facing the angled block.
Use the laser sight-equipped crossbow with normal arrows to target the metal ball to the left of the chain. You can also use pistols with the laser sight bug to conserve ammo. (If you don't have the LASER SIGHT yet, try shooting an explosive arrow in the general direction of the chains, or use the grenade gun, jumping straight up and firing at the highest point.) When you hit the ball, the gate opens.
Climb in to get SECRET #2 (51/70), a large medipack, shotgun shells, and Uzi ammo.