To go over some of the points you addressed in the PM:
I've been thinking about the Isle of Fire. My image for them was basically tall dwarves who live on an island, so some paranoid and overcomplicated defense stuff seems in order. Defensive mechanisms include:
A navy of excellent warships, to defend against invaders, prevent piracy, and enforce dominion over trade on the lake when needed. Most ships include at least a cannon/ballista/catapult, whichever is most feasible for the setting, per side, as well as some cross/bowmen to deal with enemy boarders, crew, or officers and some way of setting the enemy ship on fire. Crews also include marines trained in repelling boarding and, for the biggest and most powerful ships, a magician.
-Yeah, the navy is fine. He requested cannons for some of the ships; if no one objects, I don't really care either way, so long as you're using Middle Ages era cannons rather than those from the Age of Sail when cannons where actually practically built to be used on ships (so you'll probably need some sort of counterweight system or something).
As for the magicians, it's fine so long as they're limited to one per ship and only a few ships have them. Also, there needs to be some defined limitation to their magic (and this applies to all magic users in general). This could mean their magic is limited to a theme of some sort, is limited only to long rituals, has a weakness of some sort, has certain requirements to perform, etc. It might be a combination of several things, or include something like a needed focus or method (such as swordsmanship or martial arts or tea ceremony). Then run it by me, and, so long as it's flavorful and interesting and not overpowered (so nothing like D&D-style do-everything-else does-but-better wizards), I'll probably accept it.
Ballistae and the like bristle towards any harbor that an enemy ship could land at an deposit troops. There are enough people capable of using the weapons with some level of proficiency twice over.
Seems fine.
All adult make citizens and willing female citizens who are not in other defensive groups, such as manning the siege weapons, are trained for at least an hour or two each Saturday and Sunday, in case of invasion. There is also an official militia and the Red cliff Guard, a combination of standing army and police force. Finally, if you'll allow it, bauxite golems or similar guard important areas like the big town.
Everything except the golems is fine. You can have automated sentry turrets controlled by some vast, complex mechanical computer somewhere on the island if you want, though.