I have only visited the remains of Wallingford Castle once. What was a castle of great importance, capital of the Honor of Wallingford, is now just basically earthworks with few fragments of stone walls - not dissimilar to the fate of the castle in my home town of Pontefract, although there is even less visible walling at Wallingford.
However funding has now been made available by Historic England to conserve what does survive and hopefully to encourage greater interest in the site. This is outlined in a BBC News article, which also includes a reconstruction of the very impressive appearance of the castle in the late middle ages. This can be seen at Medieval castle ruins to undergo repairs
Wikipedia has an illustrated account of the history of the castle, which concentrates on the part it played in the conflict between the Empress Matilda and King Stephen. This can be accessed at Wallingford Castle