Ruins Barbarossakapel in Nijmegen

You're standing at the Barbarossa Ruin in the Valkhof Park in Nijmegen. That semicircular wall of tuff stone is all that's left of a chapel. It belonged to the twelfth-century Valkhof castle, the castle of Emperor Barbarossa. The half-circle is the apse, the rounded end of the chapel, almost seven metres wide. The chapel was dedicated to Saint Martin, so it's also known as Saint Martin's Chapel.