Complex sentence: German estimates were for over 30000 troops to be involved in the assault and major use of airborne forces was not an option due to the small amount of level ground around the rock and perilous air currents.
Split into simpler sentences: German estimates were for as many as 30,000 troops to be involved in the assault. Use of airborne forces was not an option due to the small amount of level ground around the Rock and perilous air currents.