The Castle seen today in Lydford was originally built in the 12th Century with modifications in the 13th Century and then again in the 17th Century. It is now owned by English Heritage and is free to visit. See below on the map marked (4).
The was also an earlier castle, made of wood and also built by the normans, all that remains of the former structure are the ringworks. As you can see in the picture below, marked (3)
There must also have been an earlier castle in Lydford, perhaps on the same site as the Norman ringwork, since it recorded in history that Aelfthryth who became Queen of Wessex was born in 945 at a castle in Lydford.
Detailed excavations and studies have been carried out. If you are looking for detailed information there is an excellent publication called Lydford Castle by A. D. Saunders which gives a detailed account of what has been found and what conclusions have been reached from them.