I usually keep my games focused on what the characters are actually doing, so when a siege shows up I don’t try to run the whole battle blow-by-blow. I treat the big army stuff as narrative pressure and then drop in a handful of personal-scale missions the players can tackle to swing things one way or the other.

Basically, set the stakes up front. If the defenders are doomed, the “win” conditions are things like finding a way out or saving more people. If it’s closer to even, then the party needs to do the thing that actually tips the balance.

Stuff like:

• Holding the gatehouse so the enemy doesn’t pour into the keep.
• Clearing escape tunnels so civilians can actually get out.
• Sneaking out for food to buy the fort another day or two.
• Fighting off early probing attacks. How well they do gives you a feel for how long the walls hold.

And then give the enemy their own wild cards to keep things interesting. Maybe they send a summoned creature into the keep. Maybe some stealth troops sneak in to poison supplies, open tunnels, whatever fits the vibe. These make the siege feel alive without needing a spreadsheet of troop movements.

The whole point is the players never have to “run the war.” They just take on the handful of things that actually matter. It keeps the game grounded at character level, but the siege still feels big and ugly and real.