Castles

You should look at Poland, has a ton of castles, including the largest castle in the world, Malbork. I think RS has a new Best of Poland Tour that includes Malbork.

If you are sincere about visiting a lot of castles, and you are committed to a Rick Steves tour, I believe your best bet might be to pick a tour that begins in Cologne, Germany - like the 2024 "Best of Germany, Austria and Switzerland" 14-day tour. This tour DOES include Burg Eltz Castle, one of Germany's most significant castles, located not too far south of Cologne near the Mosel River Valley town of Cochem. But like Rick's other tours, this one has you moving out of this area and on to some other place right away, way too quickly for true castle fans. The Mosel and Rhine River Valleys are packed with castles that you can tour, hike to, cruise to on day boats, and thoroughly enjoy over 2-4 days on your own. And by arriving in the area 2-4 days before your tour starts in Cologne, you can achieve the time you need - and you'll have a quick and easy train ride to Cologne when your tour starts.

Map of Castles in the Rhine/Mosel region and nearby

Tours to get you inside some castles:

Rheinfels in St. Goar
Reichsburg in Cochem
Marksburg in Braubach

Castle-hotels and Castle-hostels:

Auf Schoenburg in Oberwesel
Burg Liebenstein in Kamp-Bornhofen
Burg Reichenstein in Trechtingshausen
Burg Stahleck in Bacharach
Diezer Grafenschloß in Diez

Rhine Castle Trail: http://www.wandern-ohne-auto.de/Rheinburgen/Karte_Rheinburgenweg2.jpg

Rhine Castle cruises: https://www.k-d.com/en/cruises/panorama-tour-ruedesheim-loreley

There are plenty of castles in Europe. When picking a tour you need to decide on the type castle you want to visit. There are castles that are castles in name only, but are mores palaces, chateaus, or simply places where people lived. Then there are the fortified castles used to defend an area from invaders. Of course, there are those that have been restored and others that are still mostly ruin. I can’t recommend a tour since we generally have a rental car.

Look at "What's Included" for each tour, "Itinerary" (what's included and free time when you could visit additional "castles" that you research), and Tour Alum Scrapbooks for ideas.
I'm lumping regions for this post; again, check each tour for specific inclusions:
England/Scotland: Tower of London, Windsor Castle, Stanway House (functioning manor house), Stokesay Castle (interesting transition from fortified "castle" to more palace like manor home), Caernarfon Castle, Conwy Castle (and walk city walls, on your own), York (medieval city walls, walk on your own), Edinburgh Castle, Stirling Castle, Urquhart Castle, Palace of Holyrood, Blenheim Palace, Dover Castle, Tintagel Castle.
France: Versailles, Loire Valley Chateaux, Guedelon Castle (Family Europe tour includes immersive activity for kids), Carcassonne, Mont St Michel (fortified island with walkable city walls).
Germany/Austria: Neuschwanstein Castle, Salzburg Fortress (on your own, fun informational displays of medieval castle life), Hofburg and Schoenbrun Palaces (on your own in Vienna), and more castles listed in specific tours.
If by castle you mean medieval/Norman style (standing or ruined) castles, and you're willing to do a lot of the legwork for planning your own tour, then check out:
https://www.nationaltrust.org.uk/visit/castles-forts and
https://members.historic-scotland.gov.uk/places

Enjoy your trip but beware of castle fatigue. Mix things up by visiting other sites like museums and churches as well. We travel independently so I can't recommend a tour - but several castles we like are the two in Hohenschwangau, Germany, the one on the lake in Veytaux, Switzerland, the one overlooking Mozart's stomping grounds, and the what's left of the one on Inis Oirr.

Any tour is going to mix it up between castles, museums, churches, medieval cities, modern architecture, wine or beer, food experience, etc. Unless you hire a private guided tour, independent travel gets you what you want.
A boat trip on the Romantic Rhine River gives you the most castles to view per hour, but you have to make time to visit any one of them.

I'm not aware of any RS tours that include a lot of castles, or even several castles. Several of his tours include one, and maybe two, castles that are actually visited.

If you want a tour that include a lot of castle visits you could look at other tour providers that specialize in that area. Just google 'castle tours of Europe' or 'European castle tours' and see if any of them meet your expectations.

The other option is to combine a tour that includes a couple of castles with some time on your own before or after that you plan specifically to visit a few castles.

Castles are all over Europe.
One of the most concentrated area for castles is Wales and the Welsh-English border.

I have been to 17 European countries, and, as others have noted, castles abound all over Europe.

For me the top-three countries to visit just for castles are Ireland, Germany and probably England/Scotland. However, there are plenty of great castles in other countries.

What sticks out about Ireland for me is that a lot of the castles are in ruins. Those are the best to see. And many of those are what are called Norman keeps -- stub-type castles the Normans built in the medieval period. My two favorites are the spooky Minard Castle (http://www.megalithicireland.com/Minard%20Castle.html) on the remote Dingle Peninsula, Ballymote Castle in County Sligo (https://en.wikipedia.org/wiki/Ballymote_Castle) and Ballylahan Castle in County Mayo.

The OP hasn’t responded if it is a RS tour desired.

https://cadw.gov.wales/visit/places-to-visit/castles-wales
Wales is often called 'the land of castles' – there are 427 of them!

If you visit Wales you’re almost certain to see one. If you live here you’re probably familiar with quite a few. Cadw looks after 44 castles — each one is unique.

Then you have the castles on the English border with Wales.

Romania has 310 castles. Bran Castle is probably the most famous of all Romanian castles. Built by the Saxons on top of an older wooden castle erected by the Teutonic Knights, it is known worldwide as Count Dracula's mysterious residence.

Romania is also less expensive than the popular and obvious destinations, where everyone else is going, too.

Romania also has Peles Castle (which is more like the King's summer Palace in the mountains) and Corvin Castle (one of my favorites).

https://en.peles.ro/opening-hours/
http://castelulcorvinilor.ro/media/?lang=ro

There are several villages with fortified churches (mostly Saxon) in SE Transilvania in between Brasov and Sighisoara - 7 of them are UNESCO world heritage sites. Brasov is a great base to access Peles Castle, Bran Castle, and the Seven Villages.

https://worldheritagejourney.com/fortified-churches-romania/

Brasov itself has an interesting walk along the city wall and bastion towers and back through the medieval-feeling old town. Sighisoara has a cool medieval clock tower and quaint old town that leads to a fun wooden-covered stairway up to the Church on the Hill.

Wales is often called 'the land of castles' – there are 427 of them! ...
Romania has 310 castles.

Well if we are gonna pull out our castle numbers, then just to say Spain has over 10,000 castles! Many hardly visited by foreign tourists as they are located away from the beaches. A nearly 1000 year Reconquista does wonders for medieval defensive architecture lol!

The list of castles in Poland you can find at https://www.zamki.pl/?dzial=zamki. Some of them are easy available (e.g. Szczecin, Darłowo, Malbork, Kwidzyn, Lidzbark Warmiński, Olsztyn, Ciechanów, Łęczyca, Warsaw, Kazimierz Dolny, Lublin, Sandomierz, Rzeszów, Chęciny, Kraków and Wieliczka, Będzin, Wałbrzych-Książ, Legnica, Poznań, Piotrków Trybunalski), but many of them are in the provinces (e.g. Baranów Sandomierski, Niedzica, Melsztyn, Pieskowa Skała, Ogrodzieniec, Olsztyn near Częstochowa, Siewierz, Danków near Częstochowa, Bąkowa Góra near Przedbórz, Opoczno and Drzewica, Janowiec nad Wisłą (near Puławy - by bus and Kazimierz Dolny - by ferry), Czersk near Piaseczno, Liw near Węgrów, Reszel, Gniew, Kruszwica, Sieraków, Krosno Odrzańskie, Osieczna, Kórnik, Uniejów).