The OS map shows a road called 'Rhydyn Hill' skirting Caer Estyn, so I can only assume that the springs are very close by, perhaps coming out from beneath the hillfort and down to the river below.
On Rhyddyn demesne, belonging to Sir Stephen Glynne, adjoining to the Alyn, are two springs, strongly impregnated with salt; which, in dry weather, used to be the great resort of pigeons to pick up the hardened particles. These were formerly used as remedy in scorbutic cases. The patients drank a quart or two in a day; and some boiled the water till half was wasted, before they took it. The effect was, purging, griping, and sickness at the stomach, which went off in a few days, and then produced a good appetite. Dr. Short gives an instance of a woman in a deplorable situation from a scurvy, who was perfectly restored by the use of these springs.
Tours in Wales, by Thomas Pennant (1810, v2 - p54): digitised at Google Books.
Caer Estyn is more of an 'enclosure' than a full-on hillfort. It's not on a very steep hill, and the single wall didn't have a ditch. On the hill opposite are the remains of Caergwrle castle - the last proper Welsh castle, built in 1278 by Dafydd ap Gruffydd. Coflein's record hints that there might have been a similar enclosure on that hill too.