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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.