Complex sentence: Metal mirrors in this armour were considered not only as protection from cold steel and arrows, but also it's believed that mirrors could reflect evil eye, so they should be polished and worn over other armour.
Split into simpler sentences: Metal mirrors in this armour were considered not only as protection from cold steel and arrows, but also from supernatural influence. It was believed that mirrors could reflect the evil eye, so they were polished and worn over other armour.