An absolute tourist must see. Take a good three to...

An absolute tourist must see. Take a good three to five hours at least to be able to see it all and not have to rush through. The audio tour is incredibly well done, you can choose your own route or follow the recommended. Without it, you might miss quite a lot of extra information, so I would really recommend it - more so than an actual tour. When it gets crowded (and it will), it's so easy to for a guide to get drowned out whereas the audio tour will always be clear and can be rewinded if you did miss something.
FYI you can bring your own food and drinks and should take advantage of that - as with all tourist attractions, prices can be a bit hefty. Only exception in summer are the ice cream trucks - well worth the price. Strawberry clotted cream is the winning flavour!
Some of the exhibitions are not wheelchair accessible and the cobbled streets might be uncomfortable as well, but there are some you can get into (Crown jewels, at the very least). Best to inform yourself in detail on the website though.