With reference to the architectural features of Harappan cities, consider the following statements:
1. The Citadel was always walled and physically separated from the Lower Town.
2. Residential buildings were usually multi-storeyed and centered around courtyards.
3. Drainage systems were absent in smaller settlements like Lothal.
4. Bricks used throughout the civilisation followed a standardised length-to-breadth-to-height ratio.Which of the statements are correct?