Thymbra was an ancient city, as evidenced by one of the six gates of Troy being named after it. It was about 50 stadia from Troy itself, or 5-10 miles (the length of a stadia is variable).
It was at Thymbra where Achilles profaned a temple by killing a priest, and is where Kassandra received her powers of knowing what will happen but not being able to convince anyone to believe her.