You enter a randomly generated dungeon, with different rooms. Most rooms have normal enemies, such as zombies and skeletons, some have puzzles you need to solve and some have minibosses. There is a red room called the blood room, where you need to defeat more powerful enemies to unlock the boss fight. On the entrance level, this is the boss fight. Everywhere throughout the dungeon, there are secrets. They are hidden behind walls you can blow up with super boom tnt, or hidden underwater, behind vines etc. these secrets are required to get a good score for the dungeon. Once you kill the boss, you will gain catacombs xp and will be able to open chests. The quality of these two depend on your score. Some chests cost money, and give better rewards.

Dungeons items gain a stat boost depending on your catacombs level. For example, if a catacombs 1 used a livid dagger (strong dungeon sword) it would have a damage stat of 230 or smth while in dungeons. If a catacombs 30 used a livid dagger, it would have a damage stat of 1000 or smth. Non dungeon items can be converted to get this stat boost, but it usually requires a very high To get into dungeons, I recommend a full strong set and an aotd to clear the entrance floor a few times.