The city-states—five major walled cities and several smaller towns have been excavated—were all located in the three main irrigated regions of Upper Burma: the Mu River Valley, the Kyaukse plains and Minbu region, around the confluence of the Irrawaddy and Chindwin Rivers. Part of an overland trade route between China and India, the Pyu realm gradually expanded south. Halin, founded in the 1st century AD at the northern edge of Upper Burma, was the largest and most important city until around the 7th or 8th century when it was superseded by Sri Ksetra (near modern Pyay) at the southern edge. Twice as large as Halin, Sri Ksetra was the largest and most influential Pyu centre.