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Building a Castle: Ditches, Moats and Tunnels

Some defences took a lot of digging

In many castles, the ditch was a pre-existing topographical feature, but in some it had to be laboriously dug out by teams of serfs or soldiers. Elsewhere, some lords used lakes or rivers as natural water defences, while others employed earthworks experts or water engineers to create and fill their moat. (The Medieval Castle Haynes Manual)

The site was chosen with these things in mind and would often be on a hilltop or promontory to provide defences, but consideration was also given to the availability of resources: stone and trees for building materials and food and water for the workmen and eventually for the castle’s inhabitants.

The Guédelon project was sited in an abandoned quarry in miles of oak woodland, so the stone and timber they needed was right there. They also brought in water diviners who found water only 6 metres down, so they were able to dig wells.

Preparing the site

The first job was to dig the ditches and prepare the mound on which the castle would be built. The earth from the ditches and foundations was used to make the mound. Sometimes ditches had to be created through rock, and then quarry men and miners would be needed, both skilled professions which were handed down from father to son.


Caerphilly Castle

The purpose of a moat was to keep siege engines at a distance and prevent walls from being undermined. Not every castle had a moat, in fact most just had ditches. But where water was available, streams or rivers were dammed or diverted, or lakes remodelled. Often the ditchers and dykers were Flemish, with extensive experience keeping the sea at bay at home.


Swansea Castle

Miners were also used to dig tunnels. Many castles had tunnels from the keep to another safe place or means of escape in case the castles was overrun, or a tunnel to the well. There was a tunnel between Swansea Castle and St Mary’s Church nearby, which used to run through the cellars of houses right up to the 20th century. Tunnels were also used as a means to surprise attackers by coming at them from the rear.

[adapted from The Medieval Castle Haynes Manual by Charles Phillips and The World of the Castle website]

Ann Marie Thomas is the author of four medieval history books, a surprisingly cheerful poetry collection about her 2010 stroke, and the science fiction series Flight of the Kestrel. Book one, Intruders, and book two Alien Secrets, are out now. Follow her at