One carving at the Externsteine is genuinely medieval and genuinely remarkable; most of the rest of what gets claimed about the rocks is not.
What is actually old
A large relief carved directly into one of the sandstone pillars depicts the Descent from the Cross, generally dated to the twelfth century and attributed to monks connected with the nearby Abdinghof Abbey. Chambers and steps cut into the rock, along with a small round window aligned to catch the midsummer sunrise, indicate the formation was adapted for Christian devotional use across the medieval period, including as a hermitage site.
The Irminsul question
A persistent claim identifies the Externsteine as the location of the Irminsul, a sacred pillar of the pre-Christian Saxons that Charlemagne is recorded as having destroyed in 772. No archaeological evidence ties the pillar specifically to this rock formation, and the identification rests on inference rather than record; mainstream scholarship treats it as unproven rather than as a settled fact.
Reused again in the twentieth century, and again now
The site was pressed into service as a symbol of Germanic heritage by nationalist and later Nazi-era interpreters, who read invented significance into its astronomy and layout, an episode the site's earlier history had nothing to do with. Contemporary German Heathen and neopagan groups now hold ceremonies there, treating the rocks as a living ritual site independent of the disputed ancient claims, a modern practice worth naming as modern.