Dukes Johann Casimir, Ernst I and Ernst II had an approximate 50 hectare landscaped garden created around Callenberg in the 19th century. However, the park was restructured after the Second World War. Today, almost nothing of the beautiful park that the dukes had created remains to be seen. The largest areas are used for forestry and fish farming.
There is a small building by the driveway to the castle - a cottage. It was erected between 1844 and 1845 by the ducal building officer Georg Scherzer of Gotha. The building was supposed to emphasize the picturesque impression of the English-style park. A dog kennel was connected to the building. In 1950, the kennel was expanded into a publically accessible menagerie. This small zoological garden is one of the greatest attractions of Callenberg Castle. The cottage and menageries were the starting point of ducal hunts in the adjoining game park up to 1918.
A separate cemetery was built in 1944 for the ducal family. Five graves are found there: Prince Hubertus, Duke Carl Eduard, Duchess Victoria Adelheid, Princess Caroline Mathilde, and Prince Friedrich Josias. A dog cemetery was even built in 1846 close to the pheasant run for the hunting companions of Duke Ernst II. He had six of his dogs buries there. Each one was given its own headstone.