If lockdowns are an issue, you will want to think gozas & other add-ons, including Heritage items, with a few item built things to pull things together. Add-ons do not take away from the total lockdowns available in the house. (I believe the only exception are the ML furniture deeds that become containers when placed)
A keep has a LOT of bleak gray stone, so breaking up the monochrome monotony into pleasing/contrasting sections is generally a good way to go. I would advise using actual secure containers as part of the deco, that way you will be making your storage functional as well as decorative.
The pics above are a good example of this...heavy use of items that are add-ons, and they have really broken up that slab o' gray.
Here is a link to a castle/keep pic thread.
Many of the examples are very lockdown intensive, some extremely so, but you should get some good ideas from there and be able to adapt them for your situation.
-Skylark