Complex sentence: Arcadia was Gallifrey's second city fortified with four hundred "sky trenches" thought to be impregnable until an invading Dalek squadron swept them away (as seen in the 2013 minisode "The Last Day").
Split into simpler sentences: Arcadia was Gallifrey's second city fortified with four hundred "sky trenches". These were thought to be impregnable until an invading Dalek squadron swept them away (as seen in the 2013 minisode "The Last Day").