A castle is a token that can be placed on a small city (one made out of two city 'end' pieces) to enable the owner of the castle to score points for the next feature completed in the castle's 6-tile fief.
Placing[]
If you finish a 2-tile mini city you can either score the four points immediately or you can turn the city into a castle by placing one of your three castles (two in 5-6 player game) on the city and keeping the follower on it. The follower is now a lord and cannot be affected by other events to do with cities and knights.
Scoring[]
The owner of the castle will score for the next feature completed in the castle's fief. The fief comprises of the two tiles the castle is on and the 2 tiles either side which makes 6 tiles in total. The owner cannot choose which feature is scored, it must be the next one. The castle is worth 0 points at the end of the game but it gives any adjoining farms 4 points instead of the usual 3.