Kautilya's Arthashastra 2.3.7

2.3.7

vaprasyopari prākāraṃ viṣkambhadviguṇotsedham aiṣṭakaṃ dvādaśahastād ūrdhvam ojaṃ yugmaṃ vā ā caturviṃśatihastād iti kārayet -
rathacaryāsaṃcāraṃ tālamūlaṃ murajakaiḥ kapiśīrṣakaiś cācitāgram //

Above the rampart, parapets in odd or even numbers and with an intermediate, space of from 12 to 24 hastas from each other shall be built of bricks and raised to a height of twice their breadth.

The passage for chariots shall be made of trunks of palm trees or of broad and thick slabs of stones with spheres like the head of a monkey carved on their surface; but never of wood as fire finds a happy abode in it.