Book 3 was a blast! It rolls along really well in an open sandbox feel throughout the first 2/3s of the book. With my group, they were back and forth from Longshadow to the Fangwood forest during book 2 and 3. The need to resupply, purchase items, and cure conditions meant that they got involved with the diplomatic efforts involving the town defense long before they were supposed to.
After a long discussion with the players, we decided to switch out the siege with something that didn't involve the wonky way that the book wanted us to run it. I gave my players three options of handling combats while I also made several pages of city events which would correspond with the timing of the siege.
Option 1: Full Immersion- Players would operate all ally forces during each combat throughout the siege. This would take much longer than the other choices but would allow the players to feel as if nothing was left to chance in their defense of the city.
Option 2: Party Only- Players would only control combats where they assigned their characters. Multiple battles may be raging, but all conflict offscreen would be handled based on CR differences to enemy forces they faced. Players would still assign forces based on intel and their judgement but combats at the table would be limited to those their PCs were at.
Option 3: Cinematic- Players would respond to threats and assign forces where needed. Tactical decisions would quickly be made. Combats results would depend on CR differences with enemy. Combats involving PCs would consume a portion of their resources based on this CR difference when assigning their characters to a conflict. Only epic, crucial combats involving the PCs would be played out at the table.
The players were swinging between option 2 and 3. Nobody wanted to be bogged down with 8 sessions of combats (which is about what option 1 would amount to). I'd prepared for each scenario (10 battle maps, ally troops, enemy troops and monster groups) but was mostly relieved when they locked in option 3. The great thing is that the party still made important tactical decisions when allocating defenders to each conflict throughout the nights.
By the end of the second night when enemy reinforcements starting pouring through the gate outside of Longshadow the faces I got from around the table.... Wow. Silence and solemn despair. That day they made an assault on both the enemy catapults and gate. One PC died and another nearly fell. At the end though, mission accomplished!
The remaining hobgoblins pushed against the city from all sides but intelligence gathered from the command tent gave the city defenders the edge they needed to repel the final advance. By the next morning, the city looked out upon a field of dead enemies. The PCs truly deserved to be called heroes that day as they'd saved Longshadow from total defeat.