Complex sentence: Exploiting this flaw in the Sui system, Yodong and other citadels of Goguryeo continued to hold on; by June, five months after the beginning of the campaign, not a single fortress had fallen to Sui.
Split into simpler sentences: Exploiting this flaw in the Sui system, Yodong and other citadels of Goguryeo continued to hold on. By June, five months after the start of the campaign, not a single fortress had fallen to Sui.