Siege of Ani (1064)
In 1064, a large Seljuk army under Alp Arslan attacked Ani; After a 25-day siege, they captured the city. Ani was a solid and crowded city as well as a stronghold of the Christians of the time. It was very difficult to conquer Ani, which was surrounded by formidable walls. The castles were beaten with catapults and the Seljuk soldiers entered and put the entire Byzantine army to the sword. So much so that some of the soldiers could not enter the city due to the abundance of corpses. The Seljuk soldiers captured many times more than the ones they killed and sold them as slaves or used them as targets. References

Battles involving the Seljuk Empire
Sieges involving the Byzantine Empire
Sieges of the Byzantine–Seljuk wars