snip
Well, most of the ships are going OT be attached to "docks" cast from obsidian, and built in obsidian "drydocks". Thus to conserve space. Also going to probably dredge out some space, seeing as probably players are going ot demand huge ships.
12 ballista means a minimum of 36 tiles long, before accounting for any kind of command deck (either at the front, back or middle, splitting the ballista) on a 5 wide ship. Which assuming 6 tiles length of command deck, is a 42 tile long ship, plus 2 or more to make a "bow" and "stern", so probably 45-48 long..... thats a bit long, considering a map tile is only 48x48. So that might take a while.
No problems making it from steel though, the current embark, despite embarking on "bronze lake", has more than enough iron and flux stone (and goblins. Plenty of goblinite)
assuming 42 spaces of z-1 space though, that's easily enough for the booze, food, and crew space, and some form of "engine room" (might even shove a pump at the far back for the lulz.)
The armament might take a while as trade and all for scimitars, as the whale skin clothing (seas have been pretty mundane).
You didn't give it a name btw. As using taverns to name ships is brilliant. (or libraries/temples.)
The stuff onshore is already planned. A little shanty town (primarily aboveground) in wooden buildings with 1-2 tile deep basements. Paved 3 wide streets for wagons. Couple guard towers garrisoned with marksdwarves.