The Sanctuary is a Tier 4 dungeon. It is comprised of a series of floating platforms within the end, linked by portal frames. This dungeon is a good place to farm for tier 4 equipment and money.
The sanctuary is a dangerous place if you are unprepared. Since this will very likely be your first Tier 4 dungeon, it is highly recommended to have a set of Tier 3 boss armor and sword. For this dungeon in particular, a knockback sword is a must.
Players start off on a small platform, to get to the boss, take the following turns at each of the intersections: Left, Straight, Right. An easier way to remember this offhand is that after you head up the stairs after the first left, you don't head up any more stairs until you reach the Level 5 enemies. The first button you have to press will create a bridge to the final stretch. Be extremely wary on this final stretch, for this has often caused more deaths than the boss himself, Aradnias. Click the second button and you will be teleported to the fight. When you are done, either warp den or jump off.
Contrary to popular belief, there are a total of six chests in The Sanctuary instead of five. To get to the chests, take the directions at each of the intersections:
1: Straight, Right
2: Straight Straight Right
3: Straight Straight Left Left Straight, Be wary of the Level 5 mobs on the way at the end.
4: Right, Right, Right, Left.
5: Left, Left, Right, Straight.
6:Left, Straight, Right. Instead of hitting the button to fight the boss, run past it and look over the edge.
-On the platform with the Wither Skeletons on the way to the boss, if you look over the edge, you can see the main island of The End.
-This dungeon debuts the highly infamous Level 5 Skeletons, which have rightfully earned their nasty reputation in their homeland. (It has been mentioned by darawe that the skeletons were designed for this dungeon in particular.)