İstanbul is a maritime city, so it's appropriate that one of its most distinctive landmarks is on the water. The tower, which has an observation deck, small exhibition space and a restaurant, is open to the public. Purchase your ticket from the booth on the Salacak shoreline, then board one of the small boats that cross to the island every 15 minutes. Unfortunately, the views from the island aren't great, the restaurant's food is unimpressive and the tower itself isn't very interesting inside.
In ancient times a predecessor of the current 18th-century structure was used as a tollbooth and defence point; the current tower has functioned as a lighthouse, quarantine station and restaurant. In 1999, James Bond visited in The World Is Not Enough.