Complex sentence: Helepolis was the largest and most powerful siege tower ever erected; during the Siege of Rhodes, the city defenders' superior defenses and firepower stalemated the attacking army, which were being slaughtered, denied mobility and unable to mount an effective counter-measures.
Split into simpler sentences: Helepolis was the largest and most powerful siege tower ever erected; during the Siege of Rhodes. The city defenders' superior defenses and firepower stalemated the attacking army, which were being slaughtered, denied mobility and unable to mount an effective counter-measures.