Many of these cities were divided into two or more parts. Usually, the part to the west was smaller but higher. Archaeologists describe this as the citadel. Generally, the part to the east was larger but lower. This is called the lower town. Very often walls of baked brick were built around each part. The bricks were so well made that they have lasted for thousands of years. The bricks were laid in an interlocking pattern and that made the walls strong. In some cities, special buildings were constructed on the citadel. For example, in Mohenjodaro, a very special tank, which archaeologists call the Great Bath, was built in this area. This was lined with bricks, coated with plaster, and made water-tight with a layer of natural tar. There were steps leading down to it from two sides, while there were rooms on all sides. Water was probably brought in from a well, and drained out after use. Perhaps important people took a dip in this tank on special occasions. Other cities, such as Kalibang...