Hello,

Citadel is kind of modern city which has high raised buildings which is same like captial of a state and deals with most economic value and lower town has residential buildings and home business like shops are done at that place.  The ground level of citadel is higher than citadel and lower town is located at the low side of the town for which it is called lower town.

Lower town was a large settlement area during the harappa era.

For more details search on the web you will get all the details.

Hope it help!