Caislen
A “caislen” is an old Gaelic word referring to a motte-and-bailey style castle. This typically means a motte which is a raised fortification such as a stone castle on a hill or intentionally raised area of land, which is then enclosed by a walled area below such as a large fortified walled courtyard, likely with an additional ditch, moat or other barrier beyond that. This whole lower area is known as a bailey. Go look it up for yourself elsewhere, there are many historical forms of them to learn about, and a lot of variations of the spelling.
Castle Caislen
Castle Caislen is a thus far only hinted at castle inside of this game setting. The Party have no real concept of who lives in it or where it is in their campaign setting. None the less the Dungeon Master bought this domain name back in 2010 and so it doesn't go to waste we're naming a castle and a campaign after it, while keeping the game organized here too. And that's all that needs to be said about the name at this time.