As you make your way around the level, make sure to go through the corridor/room series that have the flags and rooms that point to the outside. In one of the flag rooms, there'll be a statue and a Kevlar Armor on the altar. Here, look behind the altar to spot a switch. Trigger it to make a small compartment open (the one without a candle) with the secret inside.
While traversing the corridors mentioned above, stop when you see a mirror. Look carefully at the walls just before reaching this part of the corridor: one of them will have a faint crack on the wall. Blow it up to reveal the secret.
While going around finding the switches that summon the last boss, when you activate switch #3 (it's in a bedroom), go to the opposite bedroom. Here, open the cabinets to make them lower and reveal the secret.
When you activate switch #1, look around to spot an air vent. Break the vent and enter to obtain the last secret of the Twin Dragon expansion pack.