Grosmont Castle Grosmont Castle is a ruined castle in Grosmont , Monmouthshire , Wales , very near the border with Herefordshire , England , and overlooking a bend in the River Monnow . It is generally considered to have been largely built by Hubert de Burgh early in the 13th century , on an earlier Norman foundation , but was extended in the 14th century . It is located about north - east of Abergavenny , north - west of Monmouth , and south - west of Hereford . The castle ruins are Grade I listed as of 1 September 1956 . History . Grosmont Castle is believed to have been founded as a wooden motte and bailey castle during , or shortly after , the time that William FitzOsbern was Earl of Hereford immediately after the Norman conquest of England . Earl William was killed in 1071 and his son Roger was stripped of his lands in 1075 . The powerful Marcher Lord Pain fitzJohn acquired Grosmont in the reign of King Henry I ( 1100 – 35 ) . In 1142 , it was granted to Walter of Hereford , and became part of a single lordship with Skenfrith and White Castle . Although it has been asserted that the stone castle was built at the time of Pain fitzJohn , as the centre or caput of the honour of Grosmont , records indicate that , late in the 12th century , it was probably still a timber construction . In 1201 , King John granted the lordship of the Three Castles to Hubert de Burgh , but withdrew it from him after Hubert was captured and imprisoned in France in 1205 . After the death of King John , Hubert de Burgh regained his castles in the Welsh Marches in 1219 . It was Hubert who was responsible for turning the administrative castle of Grosmont into a fortress . Royal records from when Hubert was running the government of England , show that he was undertaking building work at Grosmont between 1224 and 1226 . His work , constructing the castle in local Old Red Sandstone , gave the castle much of its appearance today . His buildings included the gatehouse , which has mostly disappeared in the last 100 years , and the three D - shaped towers in the castle ' s " enceinte " . He retained the earlier rectangular hall block , but replaced the timber defences with a stone curtain wall . In 1233 the castle witnessed the rout of King Henry III ' s army by rebel English and Welsh forces led by Richard Marshal , 3rd Earl of Pembroke , who included in their midst Earl Hubert de Burgh himself . In the aftermath of this victory Hubert was granted back Grosmont castle , and he held it until his final fall from grace in 1239 . In 1267 King Henry III granted the castle to his second son , Edmund Crouchback . Edmund ' s son , Henry of Grosmont , 1st Duke of Lancaster , was born in the castle in about 1310 . Either Edmund , or his successors early in the 14th century , undertook the conversion of the fortress into rooms suitable for a noble household . One of de Burgh ' s D - shaped towers was demolished and accommodation was built over it . The height and size the south - west tower were increased , to make it into a five - storeyed great tower or keep , the living quarters of which could only be approached via a wooden stairway to the north . Part of the reconstruction included the building of the tall octagonal chimney for which Grosmont is noted . To the east was a giant false doorway which only allowed access to the ground and first floors . The steps currently seen rising up to the castle wall walk from this doorway is the work of 20th century restorers who are also responsible for the creation of much of the double doorway into the early hall . The castle declined in importance thereafter , but was attacked in March 1405 by Welsh forces led by Rhys Gethin and including Gruffudd , son of Owain Glyndŵr . It was defended by English forces under Gilbert Talbot , 5th Baron Talbot on behalf of Sir John Skydmore ( or Scudamore ) . The siege was eventually relieved by forces led from Hereford by Prince Henry , the future King Henry V , and between 800 and 1 , 000 Welsh soldiers were killed in the battle . Grosmont Castle was abandoned by the 16th century , and , with Skenfrith and White Castle , was sold by the Duchy of Lancaster to the Duke of Beaufort in 1825 . In 1902 it was again sold , to the landowner and historian Sir Joseph Bradney , before it passed into state ownership in 1923 . Access . The site is managed by Cadw , and is open without charge on every day of the year except for 24 , 25 , 26 December and 1 January . A lane , opposite St Nicholas ' church , near the Post Office in Grosmont village , leads north to the castle .