§ Of Beleriand and Its Realms The Silmarillion

Between Dorthonion and the Shadowy Mountains there was a narrow vale, whose sheer walls were clad with pines; but the vale itself was green, for the River Sirion flowed through it, hastening towards Beleriand. Finrod held the Pass of Sirion, and upon the isle of Tol Sirion in the midst of the river he built a mighty watch-tower, Minas Tirith; but after Nargothrond was made he committed that fortress mostly to the keeping of Orodreth his brother.