Update, mild spoilers: Instead of doing the retreating battle I decided to delay opening the gate until I had fired a bunch of spells that did damage to the mobs poking thru the bars of the gate. You can’t actually shoot thru the gate, although it would make perfect sense for arrows, however you can shoot over the gate, which opens the option for some area of effect spells.
I managed to get it down to just two spiders before opening the gate. It was an easy battle at that point.
Just closed the gate at lvl 16. That was not too difficult, since I had some help with a summoned Dremora taking the agro of the mob. And they only attacked in groups of one or two at a time.
The real problem is finishing the retaking of Kvatch. I’m getting mobbed with a small army of high level foes. They wipe out the NPC helpers and then bum rush me. Not sure, but as many as a dozen were waiting for me. It’s going to be a retreating battle. Need to find a spot to camp and use long range spells and arrows.
