Two people would be required to man both of them. And I'm unsure, I think the Illusion trainer is the only one capable of alchemy besides Tolfdir. There should still be supplemental spells, however. That's what I advise you to do.
What I would do is have those mages work on enchanting the bows. Raining literal fire and ice on targets. Mages and enchanted civilian archers is a dangerous combo. Not to mention enchanted troops being spat out. Have you seen how much you decimate a bandit camp, or an entire tomb by yourself with only one enchantment on your kit? Imagine two enchantments on all possible pieces. You'd be spewing demigods, capable of ripping through the force unlike a normal soldier could ever dream of.
I believe we're forgetting that this is tactical advantages and disadvantages for crypts, ruins, and towers. Shall we discuss something that fits this format instead? How about Hag's End? Talos, want to do an assessment (like you did with Labryinthian on the first page) on it?
Markarth
Advantages: All rock. Easily defendable entrance( sort of *See later). Many towers to shoot from. Somewhat narrow path that they'll have to cross before reaching the gates( having to endure arrow fire the whole time).
Disadvantages: They've essentially trapped themselves. There IS another entrance from the Dwarven Ruin which connects to Blackreach), it comes right next to the Jarl's throne room.
Attacking: It would basically be a waiting game, maybe take some of the towers to pressure them even more, but it'd mostly be stopping supplies.
Defending: Easy defend, tough live in. You could send out patrols to pick up food, but the enemy has most likely "boxed you in" already.
*Note: I would be very curious to see if ANY military would be able to take advantage of Blackreach, it could be invaluable or a death sentence. It could be used to set up out-of-nowhere assaults on supply carriages, camps, or certain cities like Markarth.