Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. IN China, the Sui Dynasty was followed by the Tang Dynasty in 618, the year of Yeongnyu''s ascension. GOguryeo was recovering from the Goguryeo-Sui War, and the new Tang emperor was still completing its internal unification. NEither being in a position for new hostilities, Goguryeo and Tang exchanged emissaries and upon Tang''s request, conducted a prisoner exchange in 622. IN 624, Tang officially presented Taoism to the Goguryeo court, which sent scholars the following year to study Taoism and Buddhism. HOwever, as Tang gained strength, in 631, it sent a small force to destroy a monument to Goguryeo''s victory over the Sui. IN response, Goguryeo built the Cheolli Jangseong defensive wall along the western border, a 16 year project begun in 631 under the supervision of Yeon Gaesomun. DUring this time, Goguryeo continued its battles to recover lost territory from the southern Korean kingdom Silla. SIlla''s Kim Yu-sin took Goguryeo''s Nangbi fortress in 629.