Complex sentence: Tsar Alexander III made Gatchina his prime residence, after experiencing a shock and stress of his father's assassination and the palace became known as "The Citadel of Autocracy" after the Tsar's reactionary policies.
Split into simpler sentences: Tsar Alexander III, after experiencing the shock and stress of his father's assassination, made Gatchina his prime residence. The palace became known as "The Citadel of Autocracy" after the Tsar's reactionary policies.