The Deserted Keep is an abandoned keep surrounded by lava, located deep in the Wilderness, to the east of the Mage Arena. A spider web blocks the entrance to the keep, so, unless someone else has slashed it recently, you will need a knife or a weapon with a slash attack style to enter or exit the keep. An item on the tool belt will work.
There are levers in East Ardougne and Edgeville that, when pulled, teleport the players to the Deserted Keep. Likewise, there is a lever in the Deserted Keep that will teleport members to East Ardougne and free-to-play players Edgeville. If you exit the keep to the right, beware of the Chaos Elemental, and keep an eye out for Player Killers.