Kautilya's Arthashastra 2.3.6

2.3.6

pāṃsuśeṣeṇa vāstucchidraṃ rājabhavanaṃ vā pūrayet //

At a distance of four dandas (24 ft.) from the (innermost) ditch, a rampart six dandas high and twice as much broad shall be erected by heaping mud upwards and by making it square at the bottom, oval at the centre pressed by the trampling of elephants and bulls, and planted with thorny and poisonous plants in bushes. Gaps in the rampart shall be filled up with fresh earth.