You've got a dungeon, it's full of monsters... but what's going on down there?
while it can impact the setup up of a dungeon without a doubt the going's on in the dungoen will impact how play evolves and is impacted when the PCs descend the first staircase. Here's a few notes on general goings on for now:
Civil War- Two factions are at war with each other, there will be traps along seemingly open travel routes, fresh barricades, patrols, heavily armed (or dangerous) bands of combatants, frequent patrols, and sentries near borders. Delvers may find stockpiles of food and weapons, battlegrounds full of fresh corpses, an explosion of dungeon vermin feeding on the freshly dead and forces on edge that will be difficult to surprise. There should be opportunities to exploit the situation, perhaps a side can be joined or a treasure left unguarded?
Menagerie- this is a mad wizards zoo. The dungeon is here becasue a Wizard did it. the dungeon is here to either grow monsters, collect loot from hapless delvers, or hold the wizard's collection(s) of oddities. There may be clean-up crews and guards along with apprentices and colleagues on site. Oddities are unlikely to be undefended and some of the occupants may long for escape with the assistance of the PCs.
Home Sweet Home- This is where the monsters call home. There will be a dominant faction of occupants going about their normal lives all others within will be minions,guests, pests, or thieves and the dominant faction will react to them appropriately. PCs may be able to strike a deal with the occupants to clear out some troublesome creatures or find employ as mercenaries. If the outer perimeter can be breached with little incident the party should have greater opportunity for stealthy thievery with less threat from wandering monsters of unexpected sort.
Prison- It's a prison. The occupants are either jailers, prisoners, or vermin. Depending on the nature of the prison there may be little to loot and it may be possible for PCs to find themselves trapped within. There may be factions within the prison population, VIP prisoners that will offer reward to liberators and possibly future henchmen of worth to be recruited. There will be regular patrols and procedures of the jailers to deal with and the whole problem of how to get in and out without being noticed.
Mining- The dungeon is a mine currently being exploited by one or more groups. There will be invaders, claim jumpers, miners, and possibly slaves to deal with. Stockpiles of ore (or whatever is being mined) will be present as will supplies and tools for the miners. Monsters within the dungeon will be employed in mining, defense, or pests interfering with operations. Miners may be very happy to have adventurers on hand to clear out some troublesome invaders. Maybe the PCs can take over the mine for themselves?
Catacombs- the dungeon is a collection of crypts, it's a resting place for the dead. An abandoned an old catacomb is familiar dungeon-fare filled with undead, traps, and tomb robbers. A catacomb in use will have funeral processions, vagabonds hiding in secluded corners, serve as a meeting place for illicit business and may house secret cults going about unusual rites not generally approved of by society outside the catacombs. The normal users of the catacomb may well be common folk that will not take kindly to bands of "adventurers" looting the remains of their ancestors and loved ones.
while the dead are supposed to stay put in a catacomb the place is not necessarily abandoned or under-used.
Wilderness- the dungeon is a subterranean wilderness occupied by a varied host of creatures. Tribes of creatures will still be hashing out their territories and relations with other humanoids. Fearsome monsters may lurk that are unknown to most occupants and the by-ways and passage ways will not be regularly patrolled or known to any. A dungeon wilderness presents room for exploration,exploitation, establishing allies, and finding employ for PCs. There is no established order and no dominant factions.