"In Eriador Imladris was the chief dwelling of the High Elves; but at the Grey Havens of Lindon there abode also a remnant of the people of Gil-galad the Elvenking. ... Círdan the Shipwright was lord of the Havens and mighty among the Wise."
The Lindon Watchtower is a defensive and unit production structure built at settlements available for both the Imladris faction and the Arnor faction. It becomes stronger and more extensive as Cirdan reaches certain levels, increasing its strategic value. However, this means that its fate is tied to Cirdan's, so if you don't recruit or use him much, the tower will remain in its weakest form. If you recruited Cirdan, the Lindon Watchtower can also train Lindon Guardians, heroic units capable with both sword and bow. The Watchtower has a ranged attack that will automatically seek out nearby enemies.
The Watchtower levels up by one level each time Cirdan reaches level 3, level 7, and level 10.