Alt-Regensberg Castle, Regensdorf

The castle crowns a roughly 30 metre high moraine hill and occupies an irregular hexagonal platform roughly 45 metres from west to east and 40 metres from south to north, with the village of Altburg clustered on the northwestern slope. Stone construction reflects multiple building phases. Early masonry used packed boulders with horizontal mortar courses while later work employed carefully dressed ashlar blocks with raised faces, applied around the turn of the 13th century.

The principal tower is a square keep measuring approximately nine metres on each side with walls around three metres thick. Its foundations descend to about four metres below the present ground and the surviving core of the tower is filled up to an accessible level near 5.5 metres. The lowest chamber received light through narrow wall openings. The original entrance stood above ground level on the west face and was reached by a ladder and wooden platform; at a later date a staircase was built into the northwest corner to provide access. The outer casing of the keep shows the transition from rough boulder infill set in horizontal mortar courses to the refined bossed ashlar facing applied around 1200.

On the northern part of the site a rectangular foundation butts against the perimeter wall; this area housed timber stables and other outbuildings in the 12th and early 13th centuries. These wooden structures were later shifted to the south when, in the 14th century, a stone palas was built between the keep and the circular curtain wall in the castle’s northeastern sector. The palas incorporated two transverse walls roughly two metres wide on its west and south sides and was extended further to the south in subsequent work. In its final occupation phase a rainwater collection was installed on the ground floor of the palas at the east side of the keep, with a drainage channel leading outside the curtain.

The defensive curtain takes the form of a rounded stone wall that was erected at the end of the 12th century on a foundation about two metres wide. The principal gateway lay to the northwest. Behind that walled approach excavators found the foundations of a small inner passage or kennel constructed around the mid 13th century to provide secondary access. Where the 14th century palace walls pressed against the circuit, the curtain was repeatedly repaired and reinforced to bear the additional load.

Water supply and management feature prominently. An early rainwater installation about 6.5 metres across was sealed with clay and drained through gravel into a central collection chamber. When the kennel was added the oldest shallow water pit was abandoned and around that time a deep cistern roughly six metres in depth was dug into the southwest corner. The last resident fitted a new rainwater system on the palas ground floor and built a canal to channel surplus water beyond the outer wall.

The site preserves evidence of its earliest defenses. Before the stone enclosure was built the hilltop was protected by a timber palisade. Foundations in the eastern wall are notably deep and strengthened at the corners, linked to the initial arrangements for securing water. Some sections of masonry reach widths of up to four metres, reflecting the heavy rebuilding of the third major phase. Much of the stonework survives in fragmentary form; the reservoir constructed in 1919 occupies part of the medieval footprint and the ruins have been subject to repair and protective coatings after mid 20th century excavation.