Go to the "Research Center" in town. Approach the steps, then turn 90 degrees to the right before climbing up. Walk directly to the blockaded gate. Go around the blockages to reach the gate, which is locked by a pair of beams. Climb up the gate by jumping and walking forward until reaching the top. Your character cannot fall off the opposite side. Walk on top of the gate and get on the wall to the right. Walk on top of the wall, towards the church and Hammond's house. A niche will appear on the top of the wall. Step on it to reach the secret level which contains all weapons, two gold bars, a raptor, and other strange objects.