I usually cut blocks out of balsa, the appropriate dimensions & thickness for each deck level, then stack them up on top of one another. Glue 'em up, fill seams with spackle, sand, & paint. Decks are usually 1/16" aircraft plywood. I make splinter shields by cutting 3/8" strips of poster board & gluing them to the edges of the decks/platforms as needed. Dowels for masts, etc.

The big point, for me, is that superstructure does absolutely nothing for combat, so the less time, money, & trouble spent on it, the better. If it looks "OK" from about 20' out, then that's good for my purposes. I think my stuff looks better than at least half the other ships at a given battle, which is more than good enough. Combat & show are distinctly different things.

That said, last ship I built, I did the superstructure out of ABS sheet, with ABS angles as internal framing members. It was a lot more work than I usually put into superstructure, but it came out very nice, if I do say so myself! No steel balls to dig out, holes to fill with spackle, paint over, etc. although some varmint did manage to shoot off a director, which didn't float & is now lost forever (or until I get around to finding/making a replacement). I basically did it that way to make a point (to myself, more than anybody else). The entire ship was built with no wood at all, except that required by rule (i.e. the balsa skin). The "stacked blocks" method is really very easy, & pretty cheap, too.

JM