any ideas for house improving?

Agree with crysk: there's tons of stuff you would be better of after getting rid off. The pictures give an image of lavish, grand hotel, instead of a private home - even if we'd talk about a castle! The house is beautiful outside, however. Since you can't possibly run anything close to that in the game (meaning what sims are actually capable of doing, household and party sizes etc.), I'll list my opinion below:

- Nothing wrong having gigantic rooms but you have to be extra clever with furnishing to maintain a playable lot.
- If you want to make your lot shareable, I'd focus on making it playable and after that going artistic/creative: atm. there are so many kitchen appliances that playable sims will spend most their lives running back and forth that huge house as their AI isn't sophisticated enough that they would even consider using an object that is closest... nope... usually they go to the farthest. Have only 1 usable fridge and 1 stove - if you want to have more for show.. try "bugging" them by hiding something withing them that prevents their usage. I don't know what possibly does it so you just have to try out.
- If I was you, I would make the underground levels smaller and leave more room for outdoors.
- Would delete all those small bedrooms except maybe 8.
- Would leave only about max 4 employee rooms.
- I'd either use longer dinner tables with couple grid squares between them or the regular dining tables (you're currently using), leaving more grid sq. in between or put fewer chairs (i.e. instead 4 chairs, only use 2 per table and it'll leave a sq. either side empty (could put a serving plate there or a decorative object; either way, it's half less objects than you have now).
- If you put stuff in middle of the room, it looks fuller than, if you place most items along the walls.
- Make the hallways/corridors 3 sq. min. wide, so you can use carpet flooring in the middle to give a look as if having a long runner instead using rugs from objects. Best would be 4 to 5 sq. wide, where 2-3 sq from the middle would be carpet flooring and both sides have 1 sq. i.e. wood flooring. The wider corridors also efficiently eat up more space so you don't have to have so many quest bedrooms.
- Appearance of not so empty hallway/corridor: you can place a chair or two or love-seats, some paintings and/or houseplants, end tables with lamps (or floor lamps) or statues to give the space some interest. As long as you don't clutter it as the idea is to reduce the amount of objects you currently have.

Will have to continue later, if I come up with some more suggestions, but now I just simply have to make a dinner.