What is "bucu"

Bucu or Buku is a hill island surrounded by the Trave and Wakenitz Rivers in Lübeck, Germany. It is also the name of a medieval Slavic castle, now ruined, on the island. Count Adolf II of Holstein founded Lübeck on the island in 1143. The Burgkloster, or fortified monastery, of Lübeck is located atop the ruins of Bucu. "Bucu" is also the name of a hill.