Personally, King's Castle wins. The two castles are basically identical in terms of layout and scale. King's Castle's courtyard is a little longer (when closed), opens a little wider, while Black Monarch's Castle is a little wider (when closed), and opens a little narrower. But it's a very similar layout.
I never liked the black color scheme of BMC, since it looked too cartoony-evil-- I preferred the more realistic gray coloring. Also, there were some issues with detailing on BMC. The wall sections are slightly wrong, firstly (see photo
). Here's a side-by-side showing the slight variations in the wall system:
A couple other oddities that I never liked on BMC:
- The back wall has gaping holes in the battlements
- The portcullis's vertical bars aren't evenly spaced
- There are two large holes on the front corners
- The opening for the portcullis on the top platform is too large
- The jail cell's door piece looks like it was "accidentally" an extra brick too tall
Nice points about BMC:
- The forward battlements allow for passage into the central tower on each side
- Front gateway is nice and wide
- Some cool pieces (black macaroni, inverse corner slopes in gray, inverse extreme slopes, horse bardings)
Annoyances on King's Castle:
- The gateway is pretty narrow (6-wide)
- Not many special/unique pieces (white saddle and odd-sized baseplates are about it?)
Nice points about King's Castle:
- The lowered forward and rear battlements make for "rooms" underneath
- The jail cell has an actual "lock" (a bar across the door)
- There are stairs!
Anyway, for me, King's Castle is just a better castle. BMC has a few nice points, but since the two are so similar in their basic design, it comes down to a lot of small aesthetic differences, where King's Castle wins most of the time in my book.