The twin cities of Mohenjo-daro and Harappa were center of all activities. Both cities were a mile square, with defensive outer walls. Cities were divided into lower dwellings and the Citadel housed important buildings. In the excavated sites, the Harappan settlements were found built of mud bricks, burnt bricks and chiselled stones. Mud Bricks were largely used at Harappa, Kalibangan, Lothal and Banawali besides burnt bricks. The size of bricks remained the same everywhere. The ratio of brick size was 1:2:4. Other fortified sites of this culture were at Sutkogendor, AliMurad, GhaziShah and Daburkot etc.
A remarkable feature of the large urban settlements of the Indus Valley Civilization was the regularity and order in the town planning and consideration given to the civic amenities, the sewerage system and drainage. The main streets of the cities at both Harappa and Moenjodaro were generally oriented from north to south, with connecting streets running east to west, The streets of major cities such as Mohenjo-daro and Harappa were also laid out in a perfect grid pattern, The street layout showed an understanding of the basic principles of traffic, with rounded corners to allow the turning of carts easily. These streets divided the city into 12 blocks.
Except for the west-central blocks, the basic unit of city planning was the individual house. Bricks of fixed sizes were used for building while stone and wood were also used. Buildings in the lower area were rather monotonous, being mainly functional rather than decorative.
The average house in these ancient cities appeared to have stood at least two storeys high as suggested by the thickness of the enclosing wall and by remnants of wide staircases where the steps and risers still survive to considerable height from the occupation level on the ground floor. The houses were built on plinths rising above the street level with flights of steps recessed in the wall at the front door. The doors of the houses usually opened on to the side lanes rather than on to the main streets.
The existence of a theocratic and authoritarian society indicated by the presence of large and well-fortified citadels in each of the capital cities. These citadels always face west which served as sanctuaries for the cities` populations in times of attack and as community centers in times of peace. The citadel at Harappa measuring 1400 ft. x 600 ft. on mound 40 ft. high which faced foundation with brick embankment 45 ft. thick. The citadel at Mahenjo-daro included a very large building that may have been a palace.
At Mohenjo-daro, one of the most intriguing structure is the presence of a remarkable complex of buildings centering on a great bath, "built of very fine brickwork," It consists of a large rectangular pool two meters deep with steps leading into it from the narrower ends. At the foot of the stairs is a small ledge with a brick edging that extends the entire width of the pool, such that people coming down the stairs could move along this ledge without actually stepping into the pool itself. The elaborately decorated bath at Mahenjo-daro was surrounded by a cloister, which opened onto many small rooms that may have housed priests of the city`s cults.
The great bath at Mohenjo-daro had waterproofed with bitumen. Brick colonnades were discovered on the eastern, northern and southern edges. The preserved columns have stepped edges that may have been used to hold wooden screens or window frames. Two large doors lead into the complex from the south and other entrance was from the directions of north and east. A series of rooms are located along the eastern edge of the building and in one room is a well that may have supplied some of the water needed to fill the tank. Rainwater also may have been collected for this purposes, but no inlet drains are seen.
Large granaries were located near each of the citadels, which suggest that the state stored grain for ceremonial purposes, times of shortage, and possibly the regulation of grain production and sale.
The Indus civilization had an elaborate sanitary and drainage system, the hallmark of ancient Indus cities., The Authorities maintained a highly efficient drainage system. Each and every house had a connection with the main drain. These even had inspection holes for maintenance. The conduits to the main drains running through the middle of the streets below pavement level and covered with flat stones and sturdy tile bricks. The covered drain was connected to the larger sewerage outlets which finally led the dirty water outside the populated areas. The urban plan found in these cities included the world`s first urban sanitation systems. The elaborate brick-lined drainage system for the removal of rainwater is of unparalleled engineering skill.