Probably when the first defensive structure got attacked and didn't readily surrender.
First Siege of Missolonghi happened on 1822-12-31.
One of the oldest references to the mobile siege tower in ancient China was ironically a written dialogue primarily discussing naval warfare.
A siege engine is a device that is designed to break or circumvent city walls and other fortifications in siege warfare.
Siege of Suemori happened in 1584.
Siege of Aintab happened in 1921.
Siege of Bactra happened in -208.
Siege of Badami happened in 1786.
Siege of Trarbach happened in 1734.
Siege of Nicomedia happened in 1333.
Siege of Kinsale happened in 1602.
Siege of Vellore happened in 1780.
Siege of Lovech happened in 1187.