King Ludwig's castles

I don't know for sure, but I would think anything with shoulder straps would be considered a backpack. You might get away with it, but if they said no, you could have no argument.

I thought the last time I was there, 2017, there was a check room just outside the entrance area, on the left as you went through the arch. I have also been told by the TI office at the bus stop that you can leave thing there behind the desk. There are also lockers at the train station.

Update Under FAQ on the Hohenschwangau site, it says:

Regular-size backpacks and handbags can be taken into the tours. Large
luggage, hiking backpacks or military backpacks are not allowed inside
the castles and the museum.

Please note that luggage inspection on a sample basis can occur at the
entrance to Neuschwanstein Castle. This serves to ensure the security
of all our visitors.

Baggage cannot be stored in the Ticket-Center Hohenschwangau. There
are some lockers (medium size) in front of the Tourist Information in
Hohenschwangau.

So, what is the difference between "Regular-size backpacks" and "hiking backpacks"? By hiking backpack do they mean a backpackers backpack, like for someone who is camping?

I know there are no lockers at the ticket kiosk - the obvious place to have them.