Here's an update...
I know there are times I enjoy reading about people's campaigns, sharing ideas, and seeing how they eventually turned out. So in that spirit...
We gamed on Saturday, and here's how it went. The party split up and went in three groups: one group to Eltabbar, one group to Mulsantir, and one group to Tinnir.
In Eltabbar, the party uncovered the fact that Thay was moving on Rashemen because of a whole set of lies that were concocted by Tharchion/General Azhir Kren. These were directly related to the acts of the party in Rashemen (including the death of a couple of Red Wizards there). Further, they figured out that the Thayvian agent in Mulsantir (fronting as a leatherworker), was a double agent, who was also working on behalf of Azhir Kren.
In Tinnir, the party took information about the church of Talos, and the role they've played in spreading discord among the Rashemi barbarians, as proof to prevent a civil war. There, they helped to stop the in-fighting, and further, learned that the Thayvians were marching on eastern Rashemen in addition to southern Rashemen (through Mulsantir). The barbarians there followed a great leader (Fyrra Martek, leader of the Old House) to the east, to take the fight to the heart of the army.
In Mulsantir, the party aided in forging a new pact between the Hathrans and a powerful Rashemi spirit (which had been corrupted...the dragonkin spirit). A dragon disciple/monk in the party played a key role in this accord.
Finally, the party opted to take the "quick hits" approach to the ensuing battle, instead of taking on a leadership (or more direct) role in the battles. The party first, found and killed the thayvian double agent in Mulsantir (who had a gang of half-orc barbarians who were preparing to sabotage a gate to grant easy entry to the Thayvian army). Second, they went behind enemy lines to gather information on the Thayvian army (otherwise protected by powerful illusionary magic). There they learned that the thay army had brought a group of spell bombards with them, somehow. They also succeeded in destroying a small regiment of Ogres and an Ogre/Cleric (to Talos) that held a critical piece object that would be used by the Thay army to enter Mulsantir (wands of water freezing, to get across the river of Mulsantir). Finally, the party staved off an assassination attempt by a group of demons and a wererat assassin.
It was a good start, and the party really felt like they had made a significant dent in the forces of Thay.
So, next comes the siege. Most of Mulsantir (and surrounding) will be inside the walls, which will give them roughly 5,000 soldiers, and another 5,000 to 10,000 militia (barbarians/warriors). The Thayvian army consists of 5,000 lizardfolk, 2,000 undead, 2,000 goblin skirmishers, 1,000 humanoids (blooded orc, ogres, and trolls), and 2,000 cavalry (humans on fantastic beasts). There are 10 spell bombards and 8 Red Wizard Circles supporting the armies.
So what's next? What would you do if leading the armies of Thay?
-You've lost contact with your double agent in the city
-Your initial attempts to cross the semi-frozen river have met SIGNIFICANT resistance
-Pressure from the army's general is increased significantly, as they want you to finish off Mulsantir, and strike east to join that blossoming battle.