Never been in the "we" category here, it's just so obvious that ruins of any kind and even just old-ish (like within 200 years) buildings rarely look like they were supposed to. It's disturbing and sad that most people can't seem to realise that by themselves.

A big thing missing from basically any old stone construction is all the wood.
If you see holes in the walls that's probably where wood beams were inserted to form floors, and the walls would probably be lined with wood on the inside as well. And, like.. Wooden furniture lol.
Like how the fuck do you put up paintings on stone walls? If you line it with wood you can just hammer in nails like normal.

I think churches are a great way to get an idea of what castles would look like in the past: Lots of decorations and colour, whole wood structures that are just missing now, and all the stone would probably be covered in whitewash or somesuch.

There's quite a few Reconstruction Castles, newly built castles that have tried to be as historically accurate as possible

So you don't have to imagine, you can actually visit a (reconstruction of based on evidence of the time) living piece of ancient history