Oh hey, crawling in after getting sick just as all of this was getting rolling. And for the last few days I've been unsure about how to get started with the stuff I missed.

Jon has A LOT of skills related to this kind of thing: at root, he's a medieval military commander, and that was also the goal of his education. He is not an engineer or an architect, but he would have a lot of ideas about which structures are more defensible, which are less so, which ones should be shored up, things that could possibly be added to them, and so on, and if someone else is an engineer or a builder, he would be capable of collaborating with them.

His thoughts are not about potential nautilus attacks, they're more about the Null, but they're both the same in the end. If you asked him about city defense (and Nick did, in his inbox), his initial thoughts would be on making the city itself more defensible in the long term, training every willing person to fight and every unwilling person to get to a safe location if they can, etc.

In terms of what he can contribute as a single warrior: he's a very good swordsman with a very sharp quasi-magical sword.