This article may contain spoilers. Read at your own risk!
The Hell Castle is the 5th level in Block World in the Campaign. It was removed from multiplayer in the 6.2.0 update.
It is a fenced off castle with 4 towers with staircase in each one leading to the upper hallways connected to each other and hallways on the bottom level that surround the towers and connect all around them.
After defeating the Snowman Mage in the Winter Island, Newbie and the Female Survivor travel to Hell Castle via a floating sheet of ice. The Female Survivor mentioned she saw scary shadows in Hell Castle while they were going down the staircases, thinking they are still down there.
Hidden Coin and Gem.
The coin is hidden on a ledge in a window in front of one of the 4 staircases. Just turn around when you are going up one.
On one of the ends of the castle facing the two towers is a statue of a cross. Rocket Jump to the top to find the gem.
The first method is to run around the map killing of enemies before they kill you. Avoiding the mages which shoot at you ranged (killing them first).
The second and more easier way is to rocket jump onto the top of the castle, kill most of the enemies, and then come down to kill the last few.
To defeat the boss, simply use a Gadget or a Heavy weapon to kill the mages which accompany it, just keep running and kill it.
Do not get too close, as the monsters will hit you.
Kill the monsters as quickly as possible, since you earn a star by killing monsters quickly. Do not rush it, though.
Use pets, as a minor/major backup for killing.
Follow the green arrow that appears once there are 5 enemies left, in order to kill the enemies quicker.
Restore your ammunition by aiming for the head. This method is also useful for killing monsters quicker.
Use Armor. If you have a decent amount of armor, you will not have to worry about getting damaged, because you can get 3 stars even if you have damaged armor. This doesn't apply to Health.
The enemies are somewhat tough in this level, so you can equip the weapons with a decently high amount of damage.
Verify if there are enemies behind you, in order to save yourself from ambushing enemies.
Hell castle used to have two giant dead demon creatures pinned onto the two crosses at each side of the map. However, it was removed for unknown reasons, but it is most likely that the removed objects in question have offended religious beliefs.
You can rocket jump over the fence and the barrier and explore the area around the map.
Upon jumping over though, the monsters somehow exit the map and chases you. Once you cross the lava however, they'll stop chasing you and possibly wanders in the area until the player returns.
This map was taken off of Multiplayer mode, most likely prior to the 6.2.0 update, where Heaven Garden was introduced.
It is based on the Nether from Minecraft.
This map has one of the most quiet music in the game.
There was a spelling error in the story comic where the Female Survivor said "Draon's" instead of "Dragon's".
There is a sound that sounds like a train station sound in Japan.
Recently, a statue of a dead person in a cross, presumably Jesus, was removed, due to religious reasons.
The only thing that is left is the cross without a dead person.