Referring to my previous reply to "French POW's" below.
Care of Prisoners of War from whatever service came under the Admiralty and Registers of Prisoners covering all Prisoners of War held by the British [in whatever country] during the Napoleonic Wars are held at the National Archives, Kew in document series ADM 103 = Records of Medical and Prisoner of War Departments; Navy Board and predecessors: Prisoner of War Department and predecessors: Registers of Prisoners of War.
The records usually give detailed descriptions, with the places of birth, and should show circumstances of the capture and the ultimate disposal of the prisoners. Be aware that some prisoners mostly officers were given parole and allowed to reside in towns and villages across the UK.
The majority of the Registers consist of the General Entry Books kept by the those in charge of each prison hulk, depot or parole town. They contain many lists of prisoners, usually arranged by nationality or by place of confinement or parole. This makes searching for a prisoner a difficult task, in your case your looking at Falmouth.
There are no records indexed with the name Pendennis Castle, but they may be mentioned in the Falmouth records.
To search the online catalogue [PROCAT] of the National Archive, Kew. From the NA home page at http://www.nationalarchives.gov.ukhttp://www.nationalarchives.gov.uk . from red menu bar select "Search the Archives" > the catalogue. Click on large red words "Search the Catalogue" Enter the word - Falmouth; no dates and Dept code ADM103 reveals two sets of records:
ADM 103/80 Registers of prisoners of war, 1796-1801; subseries Falmouth
ADM 103/128 Registers of Prisoners of War, 1793-1797; subseries Falmouth
The records are not available to search online but you can order a copy of the relevant pages for the name you seek, visit the Kew in person or use a researcher.
Looks like any prisoners held ashore and recorded in Falmouth were moved after 1801, probably to a prison ship [hulk] all listed by name in ADM103 registers.
There are many more records covering the same period [search prisoners of war 1801-1820], but I suggest you look at the NA research guide "Prisoners of War in British Hands: 1698-1949 (Ex-PoWs and Displaced Persons, 1945 onwards)" available online [from Catalogue home page select "research guides".