I'm going to list the hidden areas I know about, simply because there's some that aren't really hidden, but unique.
- Azrael Treehouse. Password: fear the tree.
- Skylds Treehouse. Password: Rudora.
- Bombing the wall in the Battle Arena to get to the garden/farm below.
- In the same level of the super nick's costume, go in the door that leads to the level with $200. Go through the left hallway and stand in the very top left corner. There's a sign there that says "don't tell anyone about this secret spar"? Idk if this is a bug or not.
- Bomb the cliff next to revolution and go through the waterfall into the treasure cave (no treasure, because Stefan scams people) jk
- Bomb the tree in the second room of the zone. Very boring little cave.
- in the RPS building where the king is, go into the battle arena (read the sign in the hallway). Bomb the top right rocks, about 3 to the left. You get $100 in this little underground room.
- In Azrael's Nightclub in Graal city, sit on the very top table, on the far left chair. Slide down very easily. If you go too fast, you get warped to the dance floor. This room has absolutely nothing in it except an RPS hat.
- go north from the bomb and arrow shop on belle isle. It's the first building you see on belle isle. Find the large waterfall and go through it. There's an underwater Zorbi cave with an RPS hat to be won.
- In the Mad Hatters shop in SnowTown, transform into a rat and go through the flames. There is a dragon shop. Purchase a dragon to go through To NYNs magic shop.
- Bees Knees is in the trees in North Graal. Its near the _____ cafe or club, i think? I forget the name of it, but there's a little patch that indicates its presence.
- Lag Island. People think this is a glitch when it's not. They think they're being bada** when they glitch there, when really, its not illegal to be on the island. It was put there specifically to keep people from lagging the server. But if you want to go, find the witch's hut (treehouse) in SwampTown, go in and out of the door repeatedly, really swiftly and quick until you find yourself on the island.
- The building that is in the mountains leading to SnowTown (the cliff that the top of Sardons Tower rests on). In that building, go up through 2 doors I think, then go between the poles on the right. There's a locked door there. The password is: m0$tw4nt3d
Also, for people who try glitching behind counters in hopes of going through the doors back there, I think (I'm not 100% sure though) that these doors just simply lead to the "development levels". Basically, a rough draft version of the current level you're in, where staff can experiment with renovations and changes. At least, I've seen thumbs on Era before. There was a door in the blackness of a cave, which one of the PRs stated lead to the development level of the cave. The admins can simply swap the main level with the development level if they like the changes.