I'm back with news from the tower!
...I wish I had read these replies beforehand.
I approached it from the north, sneaking along the cliffs above the tower with an entourage of 16 soldiers. As I approached the sunken entrance to the tower, I got extremely lucky. The "necromancer boss" was standing just out front of the door! My archers and crossbowmen launched a barrage at him, and between the half dozen bolts and arrows his legs were immobilized. I quickly charged down the hill and decapitated him, only to be rushed by dozens of zombies! However, without their necromancer, they weren't unstoppable. The battle raged for dozens of turns, but in the end all of the zombies that came out of the tower had been struck down. I had lost 6 companions, and the remaining 10 were in various states of health, but the day was won.
Or so I thought. As I stepped foot inside the entryway of the tower, the remaining dozen or more zombies that hadn't sallied forth charged me. My companions rushed to my aid, and the resultant battle spanned the nearby mountain slopes, with many zombies and a few companions plummeting to untimely deaths. Most of the zombies were named this time. I don't know if that makes them stronger or not, but they certainly did a lot more damage. In the end, all of my companions were slain and I had to kill the last six or seven zombies myself.
With the cliffs coated in blood and gore, and the tower no longer spewing undead monstrosities, I checked the corpse of the necromancer. He was carrying an ever-so-useful treatise on another book about something. Whatever it was, it didn't appear that I could control the dead after reading it, so I resolved to go on. After all, there were no zombies left. It should be a simple matter to enter the tower and find what I seek! There was another small room attached to the back of the first, so it was there that I would search.
As I entered the doorway to the second room, countless zombies charged en masse and tore me to shreds.
My new adventurer has just set out from the capital (very near the tower) and will hopefully reclaim what was lost with his retinue of 3 soldiers. After all, the necromancer is dead. How hard can it be?
Edit: Do zombies 'respawn' or something? I've come back to the tower at least 5 times with fresh meatshields soldiers and it seems like there's always more zombies than before. The ones that were named are still dead, including the necromancer, but others keep coming...