Complex sentence: The ruined castle atop the mountain, built between 1138 and 1167 by Archbishop Arnold I of Cologne bears the same name and was originally intended for the protection of the Cologne region from any assault from the south.
Split into simpler sentences: The ruined castle, on the summit of the hill, was built between 1138 and 1167 by Archbishop Arnold I of Cologne and bears the same name. It was originally intended for the protection of the Cologne region from any assault from the south.