Complex sentence: A multinational force under the command of Lord Peterborough had landed on the Catalan coast in August intending to capture Barcelona but first had to take possession of Montjuïc Castle which was commanded by Francisco de Velasco.
Split into simpler sentences: A multinational force under the command of Lord Peterborough had landed on the Catalan coast in August, intending to capture Barcelona. However, the force first had to take possession of Montjuïc Castle which was commanded by Francisco de Velasco.