Complex sentence: By the time the first attack took place, the walls of the fort had been raised, though it was not quite finished, so as to render it difficult, if not impossible, to gain the top without ladders, even in the lowest part.
Split into simpler sentences: By the time the first attack took place, the walls of the fort had been raised, though it was not quite finished. Hence it was difficult, if not impossible, to gain the top without ladders, even in the lowest part.