Complex sentence: He decided to defend the riverbank with only a light skirmishing screen, but to strongly fortify the Seelow Heights, which rise about above the Oder and overlook the river where the "autobahn" crossed it.
Split into simpler sentences: He decided to defend the riverbank with only a light skirmishing screen. Instead, he fortified the Seelow Heights, which rise about above the Oder and overlook the river where the "autobahn" crossed it.