A change I would like to see with castles is the ability to turn off the PVP area. I'm aware that there's a card for that, but it seems to not be permanent. Maybe I'm doing something wrong, but if not, it really shouldn't be difficult to do this. Also not everyone is into PVP and for those of us who never use that area, it's just not needed. It would also be nice if it would be easier to make a pet or house guest stay in one position. I've been fighting with using the bread crumbs and it only seems to work for a little while. Why make this hard? Another feature that would be nice is when it comes to positioning items in or around the castle, we should be able to turn the item upside. We can move an item up, down, side to side, and turn it 360, but not upside down? Also it would be nice to have more furniture. There are so many items on different worlds I've seen, and I wonder why we can't buy them in that world or the bizarre, or even in the crown shop. When I'm not questing, it's nice to just decorate a castle for a while and it would help to have more options.
I'd also love to see an item limit increase at least, but personally I think the increase should be infinite, it should be your choice if you make your castle laggy for the over decoration or not, and maybe put a warning on the more intense castles. Even if you require the elixir to make your limit basically infinite, it'd be nice so I'm not left with a castle that's only got two rooms decorated thanks to a item limit.
You can effectively turn off a PVP sigil with a Castle Magic spell.
Buy a Start Detector and a Remove PVP sigil Castle Magic spell. Put the spell in the Start Detector and make the target of the spell the Start Detector.