Itokazu Castle was built on a lime stone hill and it seems like parts of their walls were of natural large rocks.
Also, the main gate has really regular rocks, which suggests that by then the Ryukyuans already started making arched doorways. This is followed after the gates at Shuri Castle though.
And this begs a question, why was the doorways at Nakijin not arched??
Present day Itokazu Castle ruins and what it used to look like here.