The Burmese then laid siege to Ayutthaya city. After heavy cannon fire put a stop to Burmese attempts at digging up to the walls, the invaders began building a bridge at Koh Keo in order to access the walls from a new direction. Maha Chakkraphat died during the course of the siege, therefore his son Mahinthrathirat ascended the throne. Frequent Ayutthaya sorties once again prevented the Burmese from finishing the construction. Bayinnaung then sent an Ayutthayan noble he held captive under the guise of a deserter. Not only was the spy allowed into the city but he was also put into a position of power. On the night of 2 August 1569, the spy opened the gates of the city, bringing its downfall. Mahinthrathirat along with his family and the nobility were captured and taken to Pegu. Mahinthrathirat died in the same year on the way.:63 Ayutthaya became a Burmese vassal state, with Mahathammarachathirat as appointed king. The Burmese took several weeks to regroup and rest having taken Ayutthaya, which allowed Lan Xang to rally their forces and plan for prolonged guerrilla warfare. The Burmese arrived in Vientiane and were able to take the lightly defended city. Setthathirath began a guerrilla campaign from his base near Nam Ngum, northeast of Vientiane. In 1570 Bayinnaung retreated, Setthathirath counterattacked and more than 30,000 were taken prisoner, along with 100 elephants, and 2,300 pieces of ivory from the retreating Burmese.. Vientiane is to the southwest of what base?