Alt-Regensberg Castle was built about the mid-11th century AD by the House of Regensberg in the municipality of Regensdorf. The decline in importance of the castle had been shown already in the Old Zürich War, when Zürich's opponent Alt-Regensberg occupied without resistance. Later the ruins served as a quarry.
The quadratic keep dates from the first construction phase. The exterior of the residential tower has been transformed remarkably over the years. The outer casing of the 11th-century structure was filled with boll stones and mounted in horizontal mortar joints strokes. By 1200 the entire outer surface was chipped away and replaced with carefully edited boss squares. The whole tower thus received a much more upmarket appearance.
In the northern part of the complex is a rectangular foundation wall leaning against the perimeter wall. Here arose in the 12th or early 13th century wooden buildings that served as stables and outbuildings. They were moved to the south and replaced by a palas in the course of the 14th century claimed the whole area between keep and circular wall in the northeastern area.References:
Sirmione castle was built near the end of the 12th century as part of a defensive network surrounding Verona. The castle was maintained and extended first as part of the Veronese protection against their rivals in Milan and later under the control of the Venetian inland empire. The massive fortress is totally surrounded by water and has an inner porch which houses a Roman and Medieval lapidary. From the drawbridge, a staircase leads to the walkways above the walls, providing a marvellous view of the harbour that once sheltered the Scaliger fleet. The doors were fitted with a variety of locking systems, including a drawbridge for horses, carriages and pedestrians, a metal grate and, more recently, double hinged doors. Venice conquered Sirmione in 1405, immediately adopting provisions to render the fortress even more secure, fortifying its outer walls and widening the harbour.
Thanks to its strategical geographical location as a border outpost, Sirmione became a crucial defence and control garrison for the ruling nobles, retaining this function until the 16th century, when its role was taken up by Peschiera del Garda.