When posing questions about Gaelic if would be best to specify Irish Gaelic or Scottish Gaelic, as they are two distinct languages.
Irish: Caisleán cois na farraige.
Scottish Gaelic: Caisteal an cois na mara.
caisteal
How do say castle in scots
caisleán ['cashlaun']
The Irish Gaelic for 'castle' is is caisleán (sounds like cash-lawn).
Gaelic is native to Ireland, Scotland and Man; Welsh is Celtic but not Gaelic. That said, the Welsh word for 'castle' is castell.
In Irish it's "turtar mara"
In Irish it is "murach" or "maighdean mhara" (sea-maiden). In Scottish Gaelic: ?
Muir is 'sea' in both Irish Gaelic and Scottish Gaelic.
muirScottish Gaelic: muir Irish Gaelic: muir or farraige
Muir (sea)
IRISH Gaelic: aigéan or bóchna
Castle in the glen
As a noun in Irish: crann seasta (champion of a cause) (in sports) curadh (warrior, hero; champion); seaimpín (champion) barrthiománaí or togha tiománaí (champion driver) Scottish Gaelic:?