The Netherbow Port was a great gateway controlling access to Edinburgh from the Canongate (then a separate burgh). In May 1544 it was attacked by the English, who blew the gates open with a culverin (a type of cannon with a relatively long-range). The Netherbow was remodelled in the 1570s, and a central tower seems to have been added around this date. The gateway was eventually demolished in the mid-eighteenth-century as it was a hindrance to traffic.
The Canmore entry for Netherbow Port is here.