The ancient walled city of Ujjain was located around the Garh Kalika hill on the bank of river Kshipra, in the present-day suburban areas of the Ujjain city. This city covered an irregular pentagonal area of 0.875 km2. It was surrounded by a 12 m high mud rampart. The archaeological investigations have also indicated the presence of a 45 m wide and 6.6 m deep moat around the city. According to F. R. Allchin and George Erdosy, these city defences were constructed between 6th and 4th centuries BCE. Dieter Schlingloff believes that these were built before 600 BCE. This period is characterised by structures made of stone and burnt-brick, tools and weapons made of iron, and black and red burnished ware.