Kautilya's Arthashastra 2.3.10

2.3.10

viṣkambhacaturaśram aṭṭālakam utsedhasamāvakṣepasopānaṃ kārayet triṃśaddaṇḍāntaraṃ ca //

Towers, square throughout and with moveable staircase or ladder equal to its height, shall also be constructed.

In the intermediate space measuring thirty dandas between two towers, there shall be formed a broad street in two compartments covered over with a roof and two-and- half times as long as it is broad.