BattleSystem video 3 was published two hours ago: melee.
Been a huge fan of 2e BS. Been playing it since 1992. Love it: kitchen sink fantasy mass battles game! I actually love the bucket-o-dice approach to combat -- I have oodles of polyhedra dice, a dice tower that can handle rolling 20+ dice at a time.
The Castles POD book has the posters inside it, as printed pages -- the posters are spread across multiple pages. The separate booklets can often be found on e-Bay, individually. I bought a copy of Drungar so I could get the supplemental BS rules, without buying the boxed set in POD.
I recommend using one of Fat Dragon Games castle PDF sets: WINTERHAWK: Castle Base Set, or BORDERLAND KEEP: Deluxe Edition (folds flat, if you make it out of cardstock). These are modular, allowing you to build as many pieces are you need (9-foot long table shown), for your tabletop BS games. All of the shown castle wall sections, towers, gatehouse, siege towers, and battering rams, were made by applying full sheet label printouts to 3mm thick cardboard, cutting and assembling them together, to make the pieces shown. They turn out very strong, very durable. They have been in use since 2015!
I recommend printing them on full sheet label paper (8.5"x11" single piece label), cut, apply to 3mm-thick cardboard from a picture frame shop (sold in large formats), which can then be cut to size. This allows you to make very sturdy, very strong, wall sections, towers, etc. Blacken the edges with either paint, or marker (can also use gray). I reinforced mine by gluing square wooden dowels inside long wall stretches: it keeps them strong, straight (non-warping), and adds decent heft to keep them from falling over easily. Cheers!