Complex sentence: One of the key locations were the Sinyavin heights which were some 150 metres higher than the surrounding flat terrain, which were one of the few dry and clear areas, and in addition provided good observation.
Split into simpler sentences: One of the key locations were the Sinyavin heights which were some 150 metres higher than the surrounding flat terrain. The heights were one of the few dry and clear areas, and in addition provided good observation.