http://nmma.nic.in/nmma/nmma_doc/Indian Archaeology Review/Indian Archaeology 1960-61 A Review.pdf
The palace and probably the towers predate ashokan periods according to this report
The architecture already has round bastions or towers, even though mathura plaque shows square bastions. either the scenes from buddhist plaques are describing buddha period architecture or its probable that these structures have been dated earlier than they really are.
or mathura carving represents stylistic/idealistic architecture not showing the ground reality.
kautilya does talk about square bastions though in his arthashastra, both semi circular and square bastions are already observed in the indus valley civilization forts.
this report does state bastions and towers were square though
Excavations At Kausambi : Sharma, G.r. : Free Download, Borrow, and Streaming : Internet Archive