Ahlsburg, Medieval castle ruin in Ilsenburg, Germany.

Ahlsburg, also called Alerdestein, is a ruined imperial hill castle on a spur with steep granite crags above the eastern bank of the Ecker, about 4 km west of Ilsenburg. The Ecker curves around the spur on three sides. Remains of foundations, ground walls and the castle ditch survive.

Emperor Charles IV granted this stronghold to the Alard brothers in 1357 through a formal deed. The fortress served as an imperial post responsible for enforcing hunting laws across the surrounding forests.

The castle's name appears in records under several forms: Ahlsburg, Alerdestein, Mahlsburg and Wahlburg. The Alerdestein form links it to the Burgdorf family, whose members Alard the Elder and Alard the Younger are named in the 1357 deed. Visitors today see only foundation remains, ground walls and the castle ditch in the birch forest.

From the former Kolonnenweg, a signpost on a right-hand bend points uphill to the Ahlsburg; the climb from the Ecker runs for about 100 m. The ruin lies on a spur above the eastern bank of the Ecker, in the northern Harz and within the Harz National Park. Only foundation remains, ground walls and the moat survive, so the ground is uneven.

The location sits on an extremely narrow ridge where the Ecker River curves around it on three sides, creating an almost fortress-like natural barrier. This arrangement made the stronghold appear nearly impregnable, even though only stone remains and far-reaching views survive today.