Complex sentence: Certainly this invasion attempt readied the troops and facilities that would make Claudius' invasion possible 3 years later (e.g. a lighthouse was built by Caligula at Boulogne-sur-Mer, the model for the one built soon after 43 at ancient Dover).
Split into simpler sentences: Certainly this invasion attempt readied the troops and facilities that would make Claudius' invasion possible three years later. For example, Caligula built a lighthouse at (modern Boulogne-sur-Mer) that provided a model for the one built soon after at (Dover).