This is what the information board says:
The core of the Heisterburg is the almost square main castle of approx. 94 by 105 m in size (0.88 ha). A surrounding rampart that is more than 2 m high and 9.5-14 m wide has been preserved, as well as a ditch that is more than 2 m deep and up to 11 m wide. In the north-west and south-east, access is via two pincer gates. The outer bailey in the north encloses an interior space of 7.4 ha. Its ramparts are almost 4 m high and 14 m wide, much more powerful than those of the main bailey. The ditch is up to 12 m wide and up to 2.5 m deep. At the end of the outer bailey there is another pincer gate, in front of it is the second outer bailey, where a pincer gate can also be seen.
Excavations took place mainly in 1891/92 and from 1929 to 1932. According to this, the fortification of the main castle consists of a mortared double-shell curtain wall with a wall backfill made of clay and slate packing. Contrary to earlier assumptions, the ditch had a U-shaped profile.
Between the wall and the moat is the berm, which prevents the wall from slipping into the moat.
Inside the main castle, several ground plans of small rectangular sunken stone houses with entrance (stairs), clay floor or stone paving have been found, but also unexplained walls of other buildings. Further investigations revealed two shafts interpreted as cisterns or wells. In the north of the main castle one found on a 24 m? Large area 20 stone packings, iron slags, iron ore residues and horseshoes, which indicate intensive iron smelting and forging activity. A rectangular pit house with plenty of pottery from the 10th century was discovered in the first outer bailey. Three late Roman coins that were found between stones of the front wall in the east wall certainly belong to a building offering. The finds from the interior itself belong more to the 11th/12th century. century
Some suspected the predecessor of the Barsinghausen monastery, founded in 1193, to be in the castle without any clear clues. A connection with possessions of the Billungers in the nearby Deister-Süntel valley is more likely. Dating to the time of Charles the Elder size is ruled out according to the current state of knowledge.