The Harappans can be called great architects and engineers because: They built massive walls and gateways surrounding the city area to protect the city from flood and control illegal trade. Most of these roads and streets were paved with fire brunt bricks. The main streets intersected at right angles, dividing the city into squares or rectangular blocks each of which was divided length wise and cross wise by lanes. The drainage system was excellent. Drains were covered and had a gentle slope so that water could flow through it. Inspection holes were provided at intervals to clean them.

All three — houses, drains and streets were probably planned and built at the same time.