After going from the stairway room to the first antechamber, there will be two rooms to your left and right. Both rooms will have a lever and some treasure chests with enchanted items. Pulling one lever will open an extension to another room, and pulling another in that extension room will lift the cage that separates the first antechamber from the hallway to the second antechamber, where the Skeleton King awaits.
In the second antechambery is an elaborate structure that is supposed to contain Leoric's body, and three rooms to the front, right, and left. There are corner nooks with Crucified Skeletons, where if you attack enough of them, they will act like a one-way lever and some more room extensions will open up, where you can find extra treasure from chests.
Since the Skeleton King constantly resurrects fallen skeletons (or apparently generates new ones out of the ground on which he stands), they are a constant source of income. Along with the newly raised skeletons come the gold, items, and occasionally even enchanted items. Going at this for many hours can yield you tens of thousands of gold.
The following monsters can be found in King Leoric's Tomb, also Skeleton King summons all of them: