You’ll likely find yourself buried in paperwork due to bureaucracy. Most castles probably fall under 'monument protection,' which means they are subject to specific building codes and require approvals from various authorities etc....
I think in Switzerland the authorities are similar ?
Heritage protection office of your region + Local building authority + Environmental protection agency/authority + in rare cases also the ministry of culture ( if you are balling )
Additionally I think work can be only be performed by certified workers .
My opinion is that you will be just wasting a lot of money and get a lot of headaches from our beloved authorities .