Kautilya's Arthashastra 2.3.16

2.3.16

prākāram ubhayato meṇḍhakam adhyardhadaṇḍaṃ kṛtvā pratolīṣaṭtulāntaraṃ dvāraṃ niveśayet pañcadaṇḍād ekottaram āṣṭadaṇḍād iti caturaśraṃ ṣaḍbhāgam āyāmādadhikam aṣṭabhāgaṃ vā //

Having made on both sides of the rampart a circular hole of a danda-and-a-half in diametre, an entrance gate (to the fort) one-sixth as broad as the width of the street shall be fixed.

A square (chaturásra) is formed by successive addition of one danda up to eight dandas commencing from five, or in the proportion, one-sixth of the length up to one-eighth.