Map – faction grouping.

There are 3 random castles on the map. Next to them are buildings that recruit level 7 creatures (random buildings). I need to connect the buildings and castles so that: Humans - the building also recruits humans. Undead - the building recruits undead. Orcs - the building recruits orcs. I don't understand how to link them - I looked at the example of the "Mysterious Valley" map, but I didn't understand. I would appreciate your help. Thank you in advance.