How was the Citadel separated from the Lower Town?

The settlements of the Harappan civilisation reveal that the cities were divided into two parts. One was the higher mound which was meant for public building, the other was the lower part of the town which had residential quarters. The higher mound is also known as the Citadel. On the citadel, buildings were constructed on mud brick platforms. The Citadel was walled and the wall separated it from the Lower Town. The Lower Town was mostly residential. It was also walled. In the Lower Town too, several buildings were built on platforms.