The Holsten Gate ("Holstein Tor", later "Holstentor") is a city gate marking off the western boundary of the old center of the Hanseatic city of Lübeck. This Brick Gothic construction is one of the relics of medieval city fortifications and the only remaining city gate, except for the Citadel Gate ("Burgtor"). Because its two round towers and arched entrance are so well known it is regarded today as a symbol of this German city, and together with the old city center of Lübeck it has been a UNESCO World Heritage Site since 1987. (source)
Thank you Enno!