The designers have added code to prevent people from creating new accounts on the older servers. If you use these links, and you select a domain where you already have an account, it should take you right to it. If you do not have an account, and the domain is one of the many that are locked out, you will be assigned to a random "new" domain instead of the one that you selected.
Each domain has a name and a number. The names of the domains are Annwyn, Arondight, Arthur, Badon, Caliburn, Caval, Celidon, Clarent, Culwich, Excalibur, Galatine, Glessic, Goswhit, Llamrei, Pridwen and Rhon.
For quick access to your domain, you can use the number of your domain at the end of this link :
For example, if you play on Badon28, the link would be http://apps.facebook.com/kingdomsofcamelot/?s=28