The Castle can be visited
Tegel Castle

Tegel Palace, also known as Humboldt Palace, is a historic building in Berlin-Tegel. It is the most significant architectural monument in the Reinickendorf district and is situated on the northern shore of Lake Tegel, surrounded by an extensive palace park. Its history is linked to the Humboldt family, who inhabited and shaped it.
History+
The origins of the palace date back to 1558, when it was initially built as a Renaissance manor house. Elector Friedrich Wilhelm of Brandenburg later had it converted into a hunting lodge. In 1766, the estate came into the possession of the Humboldt family through marriage and became their family seat, where Alexander and Wilhelm von Humboldt also lived for many years. After his mother's death in 1797, Wilhelm von Humboldt took over the palace. From 1820 to 1824, he commissioned the architect Karl Friedrich Schinkel to redesign it in the Neoclassical style. Later, Wilhelm's daughter Gabriele, married to Heinrich von Bülow, inherited the estate. The property then passed to their daughter Constanze von Heinz, whose descendants still inhabit the palace today.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://www.wehrbauten.de/thread.php?threadid=526
This description draws on the facts documented in the linked Wikipedia article; Wikipedia content is licensed under CC BY-SA 4.0. Wording and editing: Mobile History.
Location: 52.5950, 13.2764