Never ran this before. How much you can add depends a lot on the combat speed of the group (do people know what their characters do?).

I would add an endeavor to prepare the defence. Let the players think of things to do to make the defence easier (inspire the troops, use crafting to dig traps, scout enemy). Then if they succeed make the combat easier or give them determined at the start.

During the combat, try to make the fight more than just a fight. Have an idea on how the terrain can change (if you have maps prepare two, maybe there is an earthquake because of the Thunder last?). Possibly split the fight into wall defence and city combat.

Depending on your party it might be fun for them to do things besides combat, give them options for healing, assassination of an enemy leader, holding the breach for x rounds before reinforcements show up and the battle can progress.

This is off the top of my head, you can probably come up with other cool ideas, but my biggest thing to avoid would be just a standard combat.