The castle won the North West Conservation Award from the Royal Institue of British Architects (RIBA).
Working for the Duchy of Lancaster, who own the castle, BDP undertook a major repairs programme to bring Lancaster Castle back into public use.
Seven buildings were sensitively refurbished to provide offices for the client, a ticket office for tours of the castle, museum exhibition space, teaching space for Lancaster University and a cafe, which is partly housed in a new extension.
A further seven buildings were repaired to make them weathertight.
The removal of a collection of poorer quality structures cleared space for the creation of a landscaped courtyard, improving access to many of the buildings for events and celebrations in the city.
RIBA judges said the "sophisticated regeneration of a range of buildings dating back to the 12th century provides a masterclass in sensitive restoration and re-use"
They said the new would was "minimalist in nature, subtle and elegant and enhance rather than detracting from the historic setting".
Lancaster Castle’s structure dates from the 11th century and also contains Roman remains. It is Grade I listed and, with the nearby Priory Church, it stands adjacent to the remains of a Roman camp which is a Scheduled Ancient Monument.
It has been described by English Heritage as the foremost historic site in the north west.
The buildings converted to new uses were originally built as prison buildings more than 200 years ago to house the increasing numbers of criminals from the booming industrial towns of Lancashire, including Manchester and Liverpool. The castle remained in use as a prison until 2012, with no access to the public.
The 2022 RIBA North West Award winning projects are:
*Lancaster Castle, Lancaster, by BDP
*Pooley New Bridge, Penrith by Knight Architects
*Special Exhibitions Gallery – Science and Industry Museum, Manchester, by Carmody Groarke
*The Fratry, Carlisle, by Feilden Fowles
*The Old Library, Liverpool, by OMI Architects
*The Welcome Building – RHS Garden Bridgewater, Salford, by Hodder and Partners
The six projects were selected by an expert jury who visited all 10 shortlisted projects.
Regional jury chair Hazel Rounding said of this year’s award winners: "“The north west jury were united in their appreciation and admiration of this year’s regional award winners.
"The standout themes were community benefit and re-use through exceptional and trusting architect and client relationships, which ultimately created engaging, rigorous and delightful designs.
"Each scheme contributes significantly to the region in providing sustainable, accessible places for all, important in the gradual evolution of a post-pandemic world. My congratulations to these award-winning projects, which serve as exemplar architectural solutions.”
The 2022 RIBA North West Building of the Year Award, sponsored by Taylor Maxwell, will be announced on May 24 at a ceremony at RIBA’s HQ in London.
RIBA North West Award winners will now be considered for a highly-coveted RIBA National Award in recognition of their architectural excellence, which will be announced in June.
The shortlist for the RIBA Stirling Prize for the best building of the year will be drawn from the RIBA National Award-winning projects later in the year.