Windsor castle

https://www.windsor.gov.uk/visitor-information/travel-information/getting-here/by-train

I think the bus might be faster than the train-the train we took had 20 stops!

You probably went from Waterloo rather than Paddington.

We went to Paddington Station, bought our train tickets to Windsor with a transfer at Slough. This was in 2013, but don't think it was very expensive or the train ride very long. We did have lunch in a wonderful place---Two Brewers pub. It is so charming! Was built in the 1700's. It is an easy walk from the castle. Walk left from the castle on the main road. The pub is on the left.

I've been there several times, and the Green Line coach bus from Victoria Station to Windsor was the easiest I've found, fewest stops, usually fairly empty (unless there's a load of kids coming back from LegoLand :)

Really comfy, direct, very nice.
http://www.greenline702.co.uk/

Looks like one hour.
http://www.greenline702.co.uk/Documents/greenlineWindsortoLondonwebttapr18.pdf

Although I suspect that Paddington to Windsor train could be quicker.

I do know that Waterloo to Windsor takes forever, and comes in on the wrong train station farther from the castle. I did a good job failing with THAT trip that day...
:(

Have fun!

Take the train from London Paddington Station to Windsor, you will change trains in Slough, it’s a 35 minute ride. Exit the train, turn left, walk up to the street and the castle is across the street. A return ticket (round trip), cost £11.70.

We used the Paddington-Slough-Windsor/Eton Central train route last month and it was very quick ... around 30 min. I asked a similar question here before our trip and was told the best thing to do is determine from where you'll be leaving in London. We were very close to Paddington, so that was an easy call for us. If you're on the other side of town, you'll have to factor in how long it might take to get to the appropriate departure station.