Sravasti was the capital city. But Ayodhya was an important town in Kosala.
The most popular king of Kosala is Prasenjit. He was contemporary and friend of Buddha.
Initially Kosala annexed Kashi in its territory.
Sister of Prasenjit was married to Bimbisara and Kashi was given in dowry. Later Ajatasatru son of Bimbisara killed his father and sister of Prasenjit died due to grief. In retaliation, Prasenjit confiscated Kashi.
After few years Daughter of Prasenjit was married to Ajatasatru and Kashi was given in dowry.
Later on Kosala finally became a part of Magadha kingdom.