It's a gorgeous piece of art. My only caveat is this, with something that big, you effectively reduced the amount of water in your system by at least 10 gallons. You need to take that into consideration when calculating your bioload. It will also affect what fish you can stock it with. Fish like large discus, and angels run the risk of scraping its side on a corner.
One more thing, attach a piece of insulation to the back and bottom of the castle. You are going to run the risk of cracking your tank if there isn't any protection. I've seen a tank crack from a small scratch created by sand. It was a 55, no one was in the room, and it drained in less than 45 seconds.