In 645, Emperor Taizong personally led a campaign against Goguryeo by land and sea with 113,000 Tang troops plus an unspecified number of tribal auxiliaries. His noted army captured a number of Goguryeo border fortresses, including the potent Yodong (Liaodong) Fortress, which had repulsed Emperor Yang in 612 and 613. Arriving outside Ansi Fortress on July 18, Emperor Taizong prepared to meet an approaching relief army, said to number 150,000 men, in battle. Emperor Taizong ordered Li Shiji to bait the Koreans with 15,000 troops, while Zhangsun Wuji's concealed troops would ambush them from behind. On the following day, the two sides clashed and Emperor Taizong inflicted a crushing defeat on the confused Koreans, inflicting 20,000 casualties and capturing 36,800 prisoners. Emperor Taizong then laid siege to Ansi Fortress; his troops attacked the Korean fortress as many as six or seven times per day, but were repelled each time by the defenders. Unable to breach the adamant fortress despite months of siege, Tang eventually staked everything on the construction of a huge mound, designed to tower over the walls of the fortress; however, the defenders captured and successfully held the mound despite three days of frantic assaults by Tang troops. Furthermore, exacerbated by worsened conditions for the Tang army due to cold weather (and winter approaching) and diminishing provisions, Emperor Taizong was compelled to order a withdrawal from Goguryeo on October 13, but left behind an extravagant gift for Yang Manchun, the commander of Ansi Fortress.