Dungeon Defenders is a mix of an action game and a tower defense game. The basic premise of the game is you place down towers to protect an Eternia Crystal (or more than 1 in later levels) from being destroyed by monsters. There are variations on this premise, like Survival mode, where you have to protect the crystal(s) from wave after wave of endless enemies, and Challenges, which provide you with various tasks that don't necessarily follow the basic premise.
When you first start Dungeon Defenders, you must pick one of the classes to play as. Each one has distinct advantages and disadvantages over the others, so it's very important to make an informed decision. On the console versions of the game, there are only 4 classes, but the PC version of the game has an additional 8 classes, 4 of which are variants of the 4 basic classes. All of the classes have their own set of defenses they can set up, as well as special abilities they can use if the action gets a little too hectic.
Challenges are special maps with different and unique tasks given to you, and you usually get a special weapon or piece of equipment when you complete them. All challenges take place on the same maps you play during the campaign, and they unlock once you have beaten that map during the campaign.
Defeat the final boss and you should be able to find a secret room that opens underneath a newly lit neon Open sign on the second floor of the tavern It contains 10000 mana a unique weapon for every class and a secret stash that operates like a normal forge