At one end of a corridor there is a switch you need to press in order to proceed. At the opposite end of the corridor, there is a big stone cross. Destroy the cross; it topples over, and strike a hole in the floor. Jump through the hole to a secret room.

You have to work your way through three traps to proceed. There are stone signs over the doors; do the traps in the succession shown in the 'Catacombs' secret.
If you have done this right, you will be attacked by a zombie from each side when you press the upstairs switch. Enter the secret room through one of their graves, and take the reward.