Let city defense and escort happen offscreen (or onscreen, but played by computer, or simply scripted altogether).
Player controls a strike force that makes those things possible.
For example:
Escort mission.
A huge map with a road, undead base and lots of things to do. You have a hero, some troops, no base.
You can
1) Fight some forest trolls to steal their lumber mill and use lumber to build a barricade across a side road, so Undead can't attack there.
2) Sacrifice some soldiers to hide and ambush the Undead that will try to stop the caravan.
3) Fight or sneak by some bandits, steal their loot, and use it to bribe an Ogre Lord to attack Undead base.
4) Liberate some soldiers from gnolls, and either keep them with you, or send them back to town to join the caravan, increasing its chances of success.
5) Fight some kobolds, get gemstones from their cave and trade those gems to a goblin merchant for, I dunno, mechanical orcherstra with fireworks and timed mechanism, which you can set some place away from the road and it will distract the Undead when caravan will be riding by.
6) Clear the murlocks from the lake by the road, so they won't interfere with teh caravan.
7) Find some building stone in the ruins of a village and wall the entrance into a crypt so Undead can't raise a powerful sorcerer whose corpse lies there.
8) Find some healing herbs for an old woodsman to heal his wife, so he is free to go with you and see if any trees by the road are rotten enough to be quickly used to make a roadblock - and remove them.
9) Obviously kill as many undead as possible, especially necromancers and scouts, to reduce enemy force. If you're feeling ballsy you can even trigger goblin orcherstra and Ogre attack right away, and try to burn down the base in the commotion, but that's like extra-hard mode, but an insta-win too.
You have set amount of time, and each thing you do increases caravan's chance of success, and reduces damage it takes, affecting your strength/troops/resources in the following missions.
Afterwards you show a cinematic showing the caravan making its way and how Undead attacks are countered by things you set up.
City defense
Two thirds of a map are wilderness, one third is ruins of the outer city. City citadel it on the cliff, ramp leading there and ruins are occupied by the besieging force. Note that citadel and fighting itself need not even be visible. Once again you have a hero and a limited force. You're invisible in parts of the ruins, allowing you to play a stealth mission if you want. There are some important targets, like trebuchets or powerful wizards.
You can
1) Cut down trees and make roadblocks so enemies can't get reinforcements.
2) Start fires, cause diversion and disrupt the siege camp.
3) Kill enemy sappers digging a tunnel - or let them finish, kill them, and then either use the tunnel to send your warriors to reinforce the city, or use it to call a sortie out to attack the siege camp.
4) Steal explosives from ammo stockpile and destroy the trebuchets.
5) Cause loud diversions in the forest to draw some enemies away from the fighting.
6) Light smoke signals calling down artillery strikes from the city.
And so on.