From Tolkien Gateway
Calmindon was a tower built upon Tol Uinen, an island in the long bay of Rómenna that ran into the eastern coastlands of Númenor. The island was sacred to Uinen herself, and it was here that the Guild of Venturers often berthed their guild-ship Eämbar. The captain of the Guild, Aldarion, built a tall tower on the island as a warning to ships in the bay. He called this tower Calmindon, a name which is simply Elvish for 'Light-tower'.