[9-3] E1M2: Castle of the Damned

From the start, descend the stairs and jump right into the water. Locate the lit green wall ahead, shoot it to open a passage, and follow it until you surface. This secret can also be reached by jumping off the bridge with the large red symbol over it, swimming underneath, entering a small room, and walking up the ramp on the left.

Proceed forward from the start until you reach a wall. Turn right, move forward again, then turn left. You will find a bridge over a pool of water, with a large red symbol on an archway above it. Cross the bridge. To the left, you will find a Double Barreled Shotgun. Jump off and swim underneath the area where the shotgun is located. Swim under an arch and turn right. Follow the passage, keeping the bridge above you. You will reach a pool; to the left of the entrance is a wall with a symbol high up. Stand in front of this wall, and it will open, revealing 25 health, a box of nails, and a slipgate (teleporter) to the ledge opposite the one containing the Double Barreled Shotgun.

Cross the bridge marked with the red symbol. Turn right and go through the door. Turn left and climb the staircase. In the room, locate a column to your right. Approach the side of the column facing outward and push a small stone plate on it. A door to your right will open, granting you a Quad Damage item.