Complex sentence: The castles date mostly to the 14th century and were constructed by the order of King of Poland, Kazimierz the Great and are named'Eagle Nests' as most of them were located on large, tall rocks.
Split into simpler sentences: The castles date mostly to the 14th century and were constructed by the order of King of Poland, Kazimierz the Great. They are named the "Eagles' Nests" as most of them were located on large, tall rocks.