You cannot go inside the castle as it is private property owned by Jane Clark, wife of the late historian and Conservative politician Alan Clark. They do have certain days when the castle is open to the public, so you might want to click on their website to look for open days and the like. You can however go on a footpath next to the castle which gives you some lovely views of this contested place. There are a number of rather splendid walks which take in the castle and the local Brockhill Country Park (named after the former owners of the castle).
If you have enjoyed this tale of Saltwood Castle perhaps you would like to read more on the subject then here are some academic works I perused.
The Official Website of Saltwood Castle
Historic England Website on Saltwood Castle
Constitutions of Clarendon: Honour of Saltwood and the de Broc Family
Antiquities of Shropshire, Volumes 1-2 By Robert William Eyton
Kent Curiosities by John E. Vigar
Castles in Kent by David Waldron Smithers