Each dungeon map area has it's own rules and restrictions applied upon cards that can be generated around it on the game board. Some locations are therefor generally safer, others have improved chance for generating particular type of loot or hosting a custom event.
Dungeon areas are procedurally generated at the start of the Quest and may follow a Quest specific pattern. Some rooms can generate in a locked state requiring your hero to unlock them with adequate key located elsewhere on the dungeon map - but that's not nearly the end of it. Locked locations have a unique chance of procedural upgrade which will replace the location card to a unique one and determine the cards surrounding it using one of the location presets. Most unique locations will have 3 default presets ranging from lucky (treasure room or a magical library) to terrible (significant threat or unique beasts). Regardless of the kind of location generated this way it's size cannot exceed 4. Some unique cards can only be found in these special areas and while finding one is difficult enough - finding the one you're after borders with miracles.