We don't mind flying in or our of just about any city, and definitely
prefer the most economical route over the easiest
Flight routes: While it's true that fees at some airports are more expensive than others, it's the end prices that you end up comparing, and end prices can fluctuate wildly based on many other factors.
Ground travel routes: For any "extra" travel to/from airports, distance is a reasonably good rule of thumb. That said, almost any extra travel you pay for in Switzerland will be more expensive than an equivalent journey in Germany, a country where cheap alternatives are commonplace (regional and local day passes, for example.) In some towns and regions in Germany you will find that free local train and bus travel is offered just for staying there - or sometimes for staying in certain participating hotels/apartments/B&B's.
what historic castles has anyone visited that would be a better
The more modern palaces that we often call "castles" are "historic" as well, and there are dozens of them all around. If you are in Munich, Nymphenburg Palace is great option. East of Munich, on an island in Lake Chiemsee, is Herrenchiemsee Palace. In southwest Germany - to the north of Luzern and Appenzell, roughly - you will find lots of them as well:
Castles dating from the Middle Ages: Most have been destroyed or partially destroyed over the centuries. Some remain intact, like Marksburg Castle in the Rhine Valley town of Braubach, and nearby Burg Eltz (Rick Steves' favorite in all of Europe, a short train ride and a healthy walk from Marksburg.) You might have seen Rick Steves' video on the Rhine Valley, which is probably the single best place in Europe to check out this historic period:
Dozens of castles line both sides of the Rhine just south of Koblenz. Many are ruins, some have been converted into hotels/hostels, several have small museums or offer tours.
Both sides of the Rhine have attractive old-world towns with places to stay, wineries, ferry crossings, and railways that connect them. There's also a system of walking trails that connect the towns if are so inclined.
Fly into or out of Frankfurt for easiest access to the Rhine Valley (about 1 hour by train from FRA airport.) Mainz is a nice Rhine River city to stay in before/after your flight - direct trains to FRA take 20-25 minutes.