Nordborg Castle (Danish: Nordborg Slot) is a small castle which lies on the southern side of Nordborg Lake (Nordborg Sø) in Nordborg on the island of Als in Sønderborg Municipality, Denmark. The castle premises are currently used by Nordborg Slots Efterskole as a boarding school. Nordborg Castle has also been the backdrop for a number of years for Nord-Als Musikfestival. [1]