Heretic/E1M4: The Guard Tower

Yellow key
The yellow key is located at the north of the map, but you can't reach it yet.

Directly south of the yellow key is a small building with barrels and golems. On the western wall is a switch that raises a bridge to the yellow key.

Once it's raised, grab the yellow key and run before you are damaged by the crushing ceiling.

The yellow key door is directly south of the central building.

Green key
Past the yellow key gate is a tower that leads to the exit, but the exit itself is blocked by a green key gate.

The quickest way to reach the green key is by climbing the steps to the right of the yellow door, running to the south-west corner of the room, and jumping down to the floor below. The green key is located in the ground-floor doorway to the central tower, and is behind the right-hand set of barrels.

Once you have the green key, climb up the stairs, enter the tower and take a right turn. Open the door, and climb to the top to reach the exit door.

Weapons

 * Gauntlets of the necromancer is located in the first room to the left of the starting area, and is in plain sight.
 * The Etheral crossbow is located in the western section of the map. From the docks, take the westernmost building on the northern edge.
 * The claw orb is in plain sight near the yellow key.

Multiplayer

 * Firemace bay appear near the yellow key, green key, the southwest room that opens when you normally head for the green key, at the exit, or in a building south of the Etheral Crossbow.
 * Phoenix rod can be found at the secret, along with time bombs.
 * A second Dragon claw can be found near a tomb of power. They can be reached by a right-turn past the yellow key gate.
 * Hellstaff is to the east of the yellow key.

Secrets
There is one recognized secret:
 * At the start of the map, head to the left until you see a wall with a different texture. Open it, and jump into the water below to reach a teleport.  You will be brought to a different part of the map, where you can get a map scroll, morph ovum, and a bag of holding.  In multiplayer, you can also get a phoenix rod.

The following are not counted secrets, but are helpful:
 * Directly behind the start is a hidden door that contains a tome of power (and in multiplayer, a crossbow). Collecting them releases one or two fire gargoyles.