The Castle Course hasn't been heavily marketed and I am not sure it will be relative to American style marketing. In any case, I am guessing that this course will rely on overseas visitor money to be successful. Not that local or UK play isn't important, I am sure it is, but there are many golf mad folks here that simply will not pay the green fee at the Castle. Hence, I am not sure marketing to the UK market is a high priority. Most overseas visitors and folks with money who travel will already know about the Castle. It simply becomes a waiting game. That is, waiting to see if the course is any good before sacrificing a game at another trophy club. Look at all the hype Kingsbarns had. So far as I remember, it took a few years for that place to consistently fill its summer book. I visited a year or so after it opened and the place was nearly deserted in the summer. Also, the economic climate and exchange rate are not conducive to travel. Give it time. I am guessing that system of having to book one of the other courses when booking the Old will prove to be a great boost for the Castle.
Ciao