Complex sentence: The Citadel was built by the ruling Habsburg Austrians and used to shell the unruly Hungarians in their ultimately unsuccessful 1848 uprising, as it was a prime, strategic site It also saw action in the Second World War and the 1956 Hungarian Revolution.
Split into simpler sentences: The Citadel was built by the ruling Habsburg Austrians and used to shell the unruly Hungarians in their, at the end, unsuccessful, 1848 uprising. Being a prime, strategic site, it also saw action in the Second World War and the 1956 Hungarian Revolution.