The four castles are four large buildings, one at the outermost edge of Forgeheart at each cardinal direction. They are the largest buildings on the entire server by far, and serve to protect the four Forgeheart Gatehouses. The south castle is the exception, as it is actually located ontop of the South Gatehouse.
They will be listed here in an order determined by both prominence in city layout and in order of completion, though those could very well be the same.
Constructed by BlackEyeGoat and LilFizzie, both previous players who built on the server and then moved on with life & school. The castle is containing both a tram, a top yard, & an entire full decorated core with plants, fountains & walkways.
Constructed by Grabmc1112, it is currently under construction, and is the last one to be completed, as it's the last one left unfinished as of June 2014.