There are many different types of sieges, representing many different tech levels.
My favorite siege terms are "lines of circumvallation" and "lines of countervallation" wherin the enemy really wants to build a walled torus around the enemy citadel. They want walls on the inside to stop sallies from the defenders. And walls on the outside to stop attacks by wandering armies. (Kind of like wandering monsters, but bigger.)
An actual siege is a not hugely interesting war of attrition, as the defender really doesn't want to engage against a fortified attacker (otherwise it wouldn't be a siege) and the attacker doesn't have the resources for a "knock on the castle gates."
The best assumption is that attacking force and defending force have rough manpower counts that cancel each other out. Both forces also have a rate of attrition and rate of resupply. The players are best engaged trying to fiddle those rates, rather than attacking either force directly. Charging into prepared defenses is a non-epic way of dying quite horribly.
As a simple rule, players may draw upon the defending manpower count for their activities to get minions to help them. (Minions may have encounter powers like "call catapult" or may even represent artillery tasked to help the players.) Catapults are really worthless in any kind of PC level skirmish, as their Circular Error Probable in sighting in is.. depressingly low. They're fantastic at hitting prepared spots though, and can certainly feature as part of a prepared encounter by or against the PCs. Deaths, of course, directly impact the manpower count.
Of course, the opposition also can draw down their reserves. In a day-to-day sense, extra people can be out foraging and scouting. In an encounter sense, they represent extra minions on top of whatever the encounter calls for, depending on how confident the enemy commander is. A concentration of force isn't possible because it's assumed that the PCs allies are drawing attention away from the PCs.
Thus, to break a siege as defenders, the PCs will have to infiltrate the enemy camp and increase their rate of attrition past the defenders' rate of attrition. (Assassinating leadership seldom works very well, as leadership generally has stupid-good guards and is quite replaceable in a siege.)
Given that both defenders and attackers are playing the attrition game, tactics look much the same for both sides and the players can easily feature as the deciding factor in a siege without needing to introduce a MacGuffin. By having soldiers be minions and a squad of soldiers be a proper (large or huge) ally or opponent, it's possible to simulate manpower excesses.