Two more Black Knights sets were released in 1990, which were 6059 Knight's Stronghold, a set that featured three Black Knights soldiers defending a gate against a pair of Black Falcons and their trebuchet, and 6034 Black Monarch's Ghost, a set that included a lone armored knight and a glow in the dark ghost.

The Black Knights, still not formally declared as a faction, was 'merged' with the Crusaders form a combined 'Castle' side in 1990-91 (the catalogue combined the lion and wyvern motifs). (The overall structure of 6080 King's Castle is similar to the 6085 Black Monarch's Castle, which had a similar segmentation with hingebricks as well as two towers, a prison and a gatehouse on the same position.) In 1989, three Black Knights were included in the 1584 Knight's Challenge where they jousted against the Crusaders. Despite the Black Monarch Castle's different styling and emblems, it did not belong to a named faction yet so it was considered a spiritual successor to the aging 6080 King's Castle as the largest/flagship Lego Castle set. An interesting feature the castle had was that it could be opened and closed using the special 'hinge' pieces for more play in the interior. Also, the triangular dragon shields were originally invented to represent the Black Knights. This was one of the first sets to include the horse armor piece or barding. The enormous black castle was guarded by twelve soldiers, including four mounted knights. The first well known Black Knights set released was the 6085 Black Monarch's Castle in 1988, which marked the beginning of the this yet-unnamed side.

Black Monarch's Castle seen through an archway in a catalog from 1988.

Lego black falcon knight