Complex sentence: The Libava fortress was subsequently built around the city, and in the early 20th century a major military base was established on the northern edge, including formidable coastal fortifications and extensive quarters for military personnel.
Split into simpler sentences: The Libava fortress was subsequently built around the city, and in the early 20th century, a major military base was established on the northern edge. It included formidable coastal fortifications and extensive quarters for military personnel.