sadly not entirely cheap, but at least it will look good.
Sarissa precison produces in MDF a series of Roman Town Wall kits, and they just released additional parts that can be made to create Roman Mile Forts. their wall sections are perfectly sized to fit the CoK19 Siege ruleset, and their MDF kits appear to be fairly simple to assemble, since they are basically large boxes fitted together. (an advantage of roman walls)
each long wall section and gate section is 12 inches, giving the fort a footprint ideal for the CoK19 rules.
the standard Roman Mile Fort can be used, but it lacks the corner towers expected in the CoK19 rules. wasy enough to houserule if you just want the kit.
to get a fully compatible Cok19 castle, you can buy the following pieces…
1x Mile Fort Postern Gate
2x Town Wall Corner
2x Town Wall (Long)
which comes to $69, before tax and shipping. there is no ‘fighting step’ that will fit a KoW multibase but rigging up some sort of scaffold or just temp platform should be easy enough. if you are picky about the looks, you can add a few of their roman buildings right against the walls. their low workshop building would seenmwell suited to this.
alternately if you want a slightly more fancy style, you can replace the postern gate with a Mile Fort Tower Gate, or a Town Wall Gate, which while totally the same rules wise in CoK19 adds an extra tower visually to make it look more interesting. you can do similar with the wall sections, replacing the flat standard ones with Town Wall Towers. if you do both you end up with a castle that has towers every half foot or so, which would definitely be an interesting look.
this sort of ‘castle’ is ideal for Basilea, Kingdoms of Men, even Elves. and would make a good alternative to the sterotypical ‘round towers and big visible stone blocks’ look you find in most castle kits. for non-human races you should be able to customize the paint schemes and any accessories you add to fit. for example, add bits of scrap wood and various bits to the battlements to look like a bandd of Orcs took it over and ‘reinforced’ it. or paint it with a grim etherial look for abyssals, and nightstalkers, or a decaying unclean look for undead. etc.