Complex sentence: After several hours'combat the French finally retreated' without having been able to make the slightest breach'; the fort's thick walls were impervious to small arms, and even grenades proved ineffective against the fort's bombproof roofing.
Split into simpler sentences: After several hours'combat the French finally fell back' without having been able to make the slightest breach'. The fort's thick walls proved impenetrable by small arms, and even grenades failed against the fort's bombproof roofing.