Penwortham Castle was built on the south bank of the River Ribble

River Ribble

The River Ribble is a river that runs through North Yorkshire and Lancashire, in northern England. The river's drainage basin also includes parts of Greater Manchester around Wigan.-Geography:...

, at Penwortham

Penwortham

-Landmarks:Penwortham Priory was built for the Rawsthorne family and redesigned by the Cumbrian architect George Webster. The priory was demolished due to the rapid expansion of the area and the need for new housing...

 to the west of Preston, Lancashire, England, at . The site is a Scheduled Ancient Monument

Scheduled Ancient Monument

In the United Kingdom, a scheduled monument is a 'nationally important' archaeological site or historic building, given protection against unauthorized change. The various pieces of legislation used for legally protecting heritage assets from damage and destruction are grouped under the term...

.

It was built shortly after the Norman conquest as a motte

Motte-and-bailey

A motte-and-bailey is a form of castle, with a wooden or stone keep situated on a raised earthwork called a motte, accompanied by an enclosed courtyard, or bailey, surrounded by a protective ditch and palisade...

 castle by Roger of Poitou. It served to guard the estuary

Estuary

An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea....

 of the river and a ford

Ford (crossing)

A ford is a shallow place with good footing where a river or stream may be crossed by wading or in a vehicle. A ford is mostly a natural phenomenon, in contrast to a low water crossing, which is an artificial bridge that allows crossing a river or stream when water is low.The names of many towns...

 crossing it. When Roger built Lancaster Castle

Lancaster Castle

Lancaster Castle is a medieval castle located in Lancaster in the English county of Lancashire. Its early history is unclear, but may have been founded in the 11th century on the site of a Roman fort overlooking a crossing of the River Lune. In 1164, the Honour of Lancaster, including the...

, Penwortham declined in importance. Now only the mound remains.