billnikon Loc: Pennsylvania/Ohio/Florida/Maui/Oregon/Vermont

I took that trip with my Sony HX99, has a 24-720 mm lens. You will not be that close to castles when your on the water. You may be at a minimum with a 300mm lens. The image below as captured at 700mm.

cdayton

I took that trip in reverse in 2010 and used my D300 with an 18-200mm lens. I could frame the castles very well.
I’ve added a collage to give some idea of the field of view.

ksmmike

I've taken 2 Viking river cruises. One on the Danube and one on the Rhine. For the castles, you need a minimum of 200mm if you want decent images of the castles. If you can get to 300 or 400 all the better. I will say however, you can get some nice images of more of an wide angle type of image with the castle as only part of the full image with a 24-120 or a 28-200 type lens. Sometimes you don't have to have a long reach for a nice image of the castles along the river. I will admit there were a few times I did wish I had something longer than a 200mm with me.

I will also add that many of your best images will come from a lens like a 24-70 or 35 or 50 prime inside churches or wider views of streets. I found that castles along the rivers were nice to see didn't end up being my favorite images. Especially in Budapest and Amsterdam, they are beautiful cities and a smaller lens is perfect as a walk around lens. I walked around with a 58mm F1.4 most days and never took it off my camera. If you stop to see the windmills, a 24-70 will be perfect.

Don't worry as much about the castles as much you might think. There are beautiful images everywhere on those cruises in the towns.

Mike

mikegreenwald Loc: Illinois

We've had several Viking River Cruises, and live with many happy memories and slide shows, mostly for ourselves.
From the boat, castle shots need a very long lens. I had a 150-600 along and used it frequently from the boat. On the other hand, trips ashore needed standard wide angle and standard lenses - usually my 24-105 was fine. On another such trip, I'd take the same.
On a couple of occasions, we started or ended the trip with a long (several days or more), slow drive, where the 24-105 was fine. I have a much larger collection of lenses for special purposes, but I carried no others than the above, and felt it was right; less to carry, and more time to enjoy the scenery and the restaurants and visas ashore.

ncammack Loc: American Southwest (Lots of beach, no ocean)

River cruises are a lot of fun, make sure you plan for the weather!
Hope your trip is a LOT of fun and you get some great shots!

Ships Bell in Spring, Amsterdam

Toment Loc: FL, IL

billmck Loc: Central KY

Ask the same question to 100 people and you'll get more than 100 answers!

We've done several European river cruises. I use my D7100 with the Tamron 18-400 lens because I only want to deal with one lens, and that lens covers both ends of the spectrum. I generally find that most of the pictures I take on those cruises are on the wide end, but the long end works well when cruising. You also might want to take a good look at your itinerary to find out how much daytime cruising you'll be doing to get those shots of the castles along the river. Most of the time the ship moves at night.

Bill

Moondoggie Loc: Southern California

Take the longer lens. I drove along the river and used my Tamron 18-400 on my Nikon. It worked well. Have a great time

donspears Loc: Joshua, TX

We took this cruise last November and it is quite an experience. I shoot with Olympus and used the 14-150 on my EM 1X and with the 2x field of view giving a 300mm equivalent field of view it was still a little short for many of the castles. Make sure you take a fast prime as there are many night time photo opportunities that a fast prime will come in handy. Enjoy the trip.

47greyfox Loc: on the edge of the Colorado front range

My wife and I did the Grand European from Amsterdam to Budapest. Awesome trip. I had a dslr and 24-105 and a Canon SX50 bridge. Depending on how close up you want your from the river castle shots to be drives your lens need. Imho, a 200mm is too short. Think about renting or buying a longer lens like a 100-400 or more. Or… renting/buying/borrowing a bridge camera with a minimum 600mm equivalent long end. My best distant castle shots came from the SX50, which has a max focal length of 1200mm equivalent. The SX60 and SX70 stretch to 1365mm. Think long!! The only other option is severe cropping…

wdross Loc: Castle Rock, Colorado

Billnikon has shown you a photo at 700. I have basically a very similar shot, yet I only had a 24-200 angle of view. Yes, I have fewer houses in the foreground and the image is not as telephoto "flat". But I also shot it at 200 and a much wider angle as we approached and came up to it.

Take the 200 and be happy. It will save you size and weight compared to something with much more reach.

Garson Loc: Tampa, FL

I took a Rhine River cruise from Amsterdam to Basel in July. I had a zoom lens that went up to about 350m.

Nantahalan Loc: Savannah originally; western NC now

On the Amsterdam to Basel leg in 2015, I had a D7100, 18-200, 50mm 1.8, 12-24, and a small flash.

If I had to do again with that equipment, I wouldn’t change a thing, except make sure I bought 2 more 64mb SD cards and add a collapsible rubber lens hood

I used the flash inside Marksburg Castle and a few other places, maybe 6–8 times.

Because of the bus rides to sites not on the river, sometimes I used the motor drive as fast it would go. That’s because my trigger finger wasn’t fast enough!
On the bus I would have tried to eliminate reflections by using the rubber lens hood slightly against the window.

Bohica Loc: SE Coast of NC

Back in the day, US Navy anchored off a Spanish island, sat camera on ral with 50mm lens, camera was Pentax K1000, had to light a cigarette lighter in fron of lens to turn on meter. Correction, lens was a 135mm, don't remeber how fast, this was in 82. One of the best photos I ever took, won several hundred dollars in photo contest with it

imagemeister Loc: mid east Florida

Take the 200 - otherwise, rent or buy a Sony RX10m4 - 24-600mm equiv.