Before Princess Eilonwy was born, Mona had been the kingdom of the House of Llyr, the Sea-People, and the site of their stronghold Caer Colur. Separated from Mona by a flood, the ruins of Caer Colur were then located on a small island just offshore, to the northwest. This site seems to have been abandoned when the House of Llyr fell; this occurred some fifteen years before the setting of the novel. Apparently, the sea air and tides helped accelerate the deterioration of Caer Colur.