Siege as dungeon adventure

Last night I had a cool dream. I was part of a force trying to lay siege and overwhelm a darned big castle. Problem was our force while able to challenge the castle defenses wasn't large enough to overwhelm so it quickly broke down into commando raids and even worse as morale slipped some members of the invading and defending forces started thinking for themselves and taking up key locations and charging tolls for forces to pass through instead of having to fight their way through.
So lot's of conflict and confusion, lot's of potential violence and loot waiting to be claimed.

Sounds like a dungeon adventure doesn't it?

I've got to try this one. PCs as mercenaries employed as part of the besieging force. The PCs primary motivations are likely to be political allegiance and acquisition of wealth. after contact with residents who knows what complexity will get added to the PC motivations.

The defending force and location a town or city defended by extensive fantasy world castle-works; Inside the castle we have the castle lord and his henchmen, the traditional defending force, citizenry pressed to defense and many factions (guilds, thieves gangs, secret brotherhoods, temples,cults,schools) doing all they can to survive the siege. Beneath the city we can have sewers and burial catacombs, possibly older tunnels beneath and subterranean escape routes.

Most of this could be played like a dungeon but with a difference the defenders should be organized to some extent; after encounters there should be increased probability of ever larger and more competent reaction forces engaging the PCs.

As probes and raids occur along with the fatigue of war relations between friendly and hostile forces should develop. It might actually be possible to keep a villain alive for more then an encounter after exposing them to the PCs thanks to shifting tides of battle. Mercanaries and occupants will maneuver for ways to keep wealth and limb after the siege.

So politics, scouting, battle and pillage all in a contained environment with loose time limits imposed by morale,external politics and supplies.