Where did Paul find women gathered for prayer on the Sabbath at Philippi?

Answer: By a river outside the city

On the Sabbath they went outside the city gate along the river, where there was a place of prayer, and spoke to the women there.

“On the Sabbath we went outside the city gate along the river, where it was customary to find a place of prayer. After sitting down, we spoke to the women who had gathered there.”— Acts 16:13 (BSB)