Once an important border castle, and birthplace of Henry V of England, it stood until the English Civil War when it was damaged and changed hands three times before being slighted to prevent it being fortified again.

[7] Its tower shares some similarities with that of Chepstow Castle, another stronghold built for FitzOsbern further south, at the lower end of the River Wye.

[12] The turmoil and conflict in Wales during the ten years of the Owain Glyndŵr rebellion did not directly affect Monmouth Castle as it was a stronghold of the region and lesser targets presented themselves more readily to essentially a guerilla army.

[13] Over the centuries, as its defensive function diminished, the outer bailey of the castle became increasingly used as a market place, later (and now) known as Agincourt Square.

[14] In the tumult of the English Civil War, Monmouth Castle changed hands three times, finally falling to the Parliamentarians in 1645.

The demolition took months, and Pye's diary also recorded the collapse on 22 December 1647; "about twelve o'clock, the Tower in the Castle of Monmouth fell down, upon one side, whilst we were at sermon".

[20] William Gilpin, visiting Monmouth on his Wye Tour, described the castle's decline by the end of the 18th century; "The transmutations of time are often ludicrous... formerly the palace of a king, and birth-place of a mighty prince: it is now converted into a yard for fatting ducks".

[21] Great Castle House was built in 1673, on the site of the old round tower, by Henry Somerset, 1st Duke of Beaufort.