Süpplingenburg is located in Lower Saxony in the district of Helmstedt between Helmstedt and Königslutter on the river Schunter. Today's renovated church of St. Johannis is located on the site where a moated castle originally stood, built towards the end of the 10th century. The originally square-shaped complex measured 70x70 m and was surrounded by a wall, moat and rampart. The core castle had a bower and a residential building. St. John's Church with a cloister, a Romanesque building at its core, stood on the castle grounds. The moated castle was the ancestral seat of Emperor Lothar III, who founded a collegiate monastery here in 1130, which was handed over to the Knights Templar under Henry the Lion.
After the dissolution of the Knights Templar, the convent buildings were handed over to the Order of St John, which owned them from 1357 to 1820.
In 1870, the few remains of the former castle and moats were razed to the ground. But the collegiate church has been preserved.
Individual parts of the church, the outer walls, chancel parts and the destroyed crypt date back to an earlier time, perhaps to that of Emperor Lothar III or his father Count Gebhard. There are still beautiful round arch friezes on the exterior.
Some remains of the former ancestral castle of Emperor Lothar III lie hidden in the ground of the former domain courtyard, as was proven in 1964.
In search of Gertrud:
Her father Lothar was born here in 1075. After becoming Duke of Saxony in 1106, he was elected king and crowned in 1125. In 1133 he finally became Emperor of the Roman Empire of the German Nation. He died in 1137 and was buried in the imperial cathedral in Königslutter.
His daughter Gertrud is not in the Basilica of St. Johannis in Süpplingenburg.
Good luck with your further search!