Dungeons are divided into levels. The higher the mana in the environment most likely indicates how high the level of the dungeon is. The different levels of the Dungeons are divided into tiers. Most Dungeons in kingdoms are low level. Mid tier levels are usually reserved as the royal dungeon for tier 2 and 1 kingdoms. The dungeon that was reserved for the public was a low level tier 1 dungeon which was the best low level dungeon. As expected of one of the best tier 1 kingdoms. The dungeon for the public in Noah's kingdom was a low level tier 3 dungeon.
Dungeons work by capturing different monsters and imprisoning them with a blood contract. Of course the dungeon itself cannot move on its own but has a guardian who helps it in exchange for some of the benefits given to it by the dungeon. It filters the impure mana released as byproduct from the monsters in it and purifies them with its core making it become better.
Of course it also extracts mana from its surroundings using them to increase its strength. The process might be slow as Dungeons have to share most of their gains with their guardians but what Dungeons lack the most is time. To capture a dungeon requires a Saint as all Dungeons are Saint no matter the tier. But since they lack fighting ability they use their ability to manipulate laws to their advantage. Immediately danger is sensed they manipulate space bringing their guardian who are usually high level saints mostly wyverns and firebirds straight to them. Since everyone strong enough to try capture dungeon knows if the fact no one bothers anymore.
A dungeon will also have extremely good resources because the density of mana in a dungeon is always high. It never allows anyone to share though as immediately you try meditating a monster always come as a distraction. So metals in the environment would turn to better metals. Mithril, Prometheum, Astral silver and even Adamantine can be found. Of course the better the tier the better the metal. Even plants important to alchemist could be found in dungeons.
What made Dungeons more attractive was that there was never shortage of monsters. Some monster part and the monster core could be used by alchemist. Monster fangs, bones, hooves could be used to make weapons. Even their blood are useful for experiments by humans. This just makes adventures risk their all even when knowing how dangerous Dungeons are. Even the Mithril sword used by an adventurer 5 years before could have increase in tiers by being exposed to the amount of mana that is in a dungeon.
Each floor in a dungeon is connected to each by a door. This door is usually protected by a monster stronger than the monsters on the same floor while a little strongee than the monsters on the next floor. Usually the species on the floor are the same so that the infighting could be lesser. The higher the tier, the higher the number of floors. A low level tier 8 dungeon has 7 floors while a low level tier 1 dungeon has 15 floors.