Question about backpacks while visiting Buckingham palace and Windsor castle

One reason for having to carry backpacks is that they hit other visitors (when the owner turns) at crowded sites. I don't have a problem with that rule. It's in effect at many museums. It's not just about the labor (and line delays) to search them.

I see that the Security page for Windsor mentions "large backpacks". That suggests that it will be hard to get a definitive answer to your question, even if a later reply comes from a recent visitor.

https://www.rct.uk/visit/windsor-castle/practical-information-for-visiting-windsor-castle#/#security

I was at Windsor 3 months ago (the Palace didn't open for tours until mid-July) and wore a Patagonia Atom sling bag on my back for the whole visit. I passed through the security check point with no issues. It has a slimmer profile than a backpack and I was able to carry everything that I needed for my visit.

There is a place at Windsor castle where you can check a bag if you need to. I saw people checking full size suitcases, so I’m sure you could check a backpack there.

The size limit for back packs that can be taken inside at Buckingham Palace is 45cm × 20cm × 30cm as a possible guide for Windsor. As mentioned above Windsor does have onsite storage.