Complex sentence: The gorge protected the Roman flanks from attack while the forest would impede approach from the rear; this would prevent Boudica from bringing considerable forces to bear on the Roman position, and the open plain in front made ambushes impossible.
Split into simpler sentences: The gorge protected the Roman flanks from attack, while the forest would impede approach from the rear. This would have prevented Boudica from bringing considerable forces to bear on the Roman position, and the open plain in front made ambushes impossible.