Like most other minor castles I've posted about, the beginnings of the Sichelnstein are shrouded in obscurity and some names that may have been local legend rather than historical. To make things worse, the main source about the castle, the village website and guidebook, get some basic things wrong like misdating the Battle of the Lechfeld (it took place 955, not 933) and burying Otto the Quarrelsome in the wrong place, so I take the information it provides with a spoonful of salt.
The Saxon nobles Asic and Billing mentioned in Carolingian chronicles (811, 813), who fled from their own people and established local holds in the Frankian lands between Kassel and Göttingen, can not be connected with the later Sichelnstein family.
The first person we can trace with more surety is one Wittilo who fought bravely at the Battle of Riade (933, also known as Battle of Merseburg, against the Magyars). King Heinrich I rewarded Wittilo by granting him the Sichelnstein as fief, and dubbing him. The ceremony took place during a diet in the palatine castle of Grona near Göttingen (nothing much remains of it, alas). So Wittilo was likely a ministerialis, a member of the legally unfree but still high ranking men serving in administrative functions at the royal court and as heavy cavalry in war. Heavy cavalry secured the victory at Riade which would have given Wittilo his chance to shine. To grant a ministerialis a fief was indeed a reward, though it is possible that Wittilo already had the usufruct of the land to pay for his armour, and he may have taken the name of it. An additional dubbing would enhance his status to free knight.
The castle would not have been the present stone-walled building, but fortified by an earth wall and timber palisade, though the trench may have existed already. The houses were likely half-timbered; perhaps on stone foundations.
The feudal relationship was renewed under Otto I after Heinrich's death in 936, which means the fief was not allodial and likely indeed came to Wittilo first, and not to some obscure Carolingian ancestor.
The Sichelnstein family has left a few traces in chartes, several of them issued in Corvey monastery, so there may have been a connection (probably younger sons ending up there a few times) but nothing spectacular. When the empress Kunigunde, wife of Heinrich II, founded the Benedictine abbey in nearby Kaufungen in 1017, one Bardo of Sichelnstein was among the witnesses. Today, Kaufungen lies in Hessia, but at the time Hessia didn't exist as political entity (the landgraviate of Hessia was created in 1292); the land was likely crown land.
The next fact we can prove is that the last of the Sichelnstein line, one Count Bardo, died in 1239 and was buried in Wahlshausen monastery at the Fulda river; the church still exists today. He may have been the same who killed his wife in 1189 but we can't say for sure. The Bardo who killed his wife had to answer at the diet of Fulda where King Heinrich VI (the son of Friedrich Barbarossa) first had him condemned to death but pardoned him to stay a prisoner at the monastery of Corvey. Bardo may have later returned to Sichelnstein, or the Bardo then living there was a relative. But the questionable website got one thing wrong: that King Heinrich gave the Sichelnstein fief to Heinrich the Lion. The two men did reconciliate at the diet of Fulda after Heinrich the Lion's return from exile, and he was given back the allodial possessions of the Welfen family, but he was not granted any new fiefs. So if he got the Sichelnstein lands and castle, they must have been part of his allodial possession at that point (he'd likely confirmed the Sichelnstein rights to hold the fief from him). Because of the paucity of surviving documents, we can't trace the exact time when the Sichelnstein came into the hands of the Welfen
The Sichelnstein lands were definitely in possession of the Welfen family in 1372, when Duke Otto I of Braunschweig-Göttingen (our friend Otto 'the Quarrelsome') who had inherited the patchwork of Welfen lands in Lower Saxony, fortified the castle during his war with the landgrave of Hessia. Otto's mother was the daughter of Landgrave Heinrich II of Hessia who left no male heirs in direct line, so Otto claimed the lands. But so did several others, esp another nephew of the late landgrave Heinrich, and war broke out. Otto lost that war.
The Sichelnstein remained in Welfen hands. 1379, it was given to Otto's second wife Margarethe of Berg as dowry and widow seat. Margarethe survived her husband who died in 1394, by almost 50 years and lived most of the time in Castle Hardeg, but she visited the Sichelnstein a few times.
Later, the castle came into possession of the landgraves of Hessia. Maybe Otto's successors sold it or pawned it out; money was pretty tight in that branch of the Welfen family. It was probably damaged during the Thirty Years War and then abandoned, like so many other castles. Later it was used as quarry by the surrounding villages where you can hunt the stones in various old buildings.
Today only the lower part of the curtain wall remains, which is still a formidable sight. The interior contains the set up of a wooden stage and can be protected by a canvas roof; the castle is used for concerts in summer.
The castle had once been surrounded by deep trench and only accessible via a drawing bridge that lead to the only gate in the east wall. Parts of the trench can still be seen at the western side of the castle; it still looks difficult to get across.
The groundplan of the castle shows an unusual horseshoe shape with the eastern side being straight and the western front curved. The curtain wall, built of basalt stones from the nearby Staufenberg Hill, still rises to 7-10 metres. Since it has no windows and the first traces of holdings for beams and a fireplace are at 10 metres heigth, the castle must once have been much higher, probably with half-timbered buildings like a palas sitting atop the stone walls like in this example (Falkenstein Castle in the Harz Mountains). Whether or not there had been towers can't be said for sure, but there likely was a battlement at the level of the stone walls since there are remains of arrow slits in some places.
The Sichelnstein was not a very large castle, but obviously strong enough to serve Otto the Quarrelsome as basis for his war with Hessia, so it must have looked more forbidding once, and you could cram a garrison inside if the men weren't wild about comfort.
Since the door was locked I could only peek through the grille to get a view of the empty interior. The height may have made up for the limited ground space.