15 hour layover tour close to Frankfort airport.

Well, there are layovers and then there are layovers. Arriving at 21:00 in the evening and then flying out again at noon the next day means, no, you would not get to see a whole lot. Arriving at 0500 in the morning and flying out at 22:00 that night, means that you can see a bit. Are you arriving from another country in Europe or from the states? Are you flying to another country or are you flying back to the states? This makes a difference in how early you need to be back at the airport

There is a ton of historic stuff to see in Frankfurt and in Mainz. A 1000 year old Cathedral, or a cathedral where the Emperors were elected for over 700 years and crowned for 230, a couple of 800 year old churches, Roman ruins, one of the most beautiful opera houses in the world, 700 year old watch towers, 800 year old ghetto walls, Holocaust Memorials, but no castle.

Bad Homburg has a palace and that is just a 30 min. ride on the S-bahn from Frankfurt. You might want to get out a map of the Frankfurt area and spend a little time on here learning about Germany to see where it might be possible for you to go.

I am assuming that you are flying out of the U.S. and would arrive in Frankfurt fairly early that morning. If so and you do your homework ahead of time, you should have enough time for a quick trip to the Rhine and return to catch your flight. Rhein-Main Airport is on the west side of Frankfurt, with a clear shot at the most scenic stretch of the Rhein. There is a train station under the airport, with local trains stopping at small towns on either side of the river every few miles. You could probably make it to St. Goar or Koblenz within two hours if you want to go that far. Pick a location, check departure and arrival times coming and going and you should have a couple of hours or more left over to explore the castle of your choice. Driving is also easy but more costly and probably unnecessary. Just remember to be back two or three hours before your flight is scheduled to leave. Security checks alone can take an hour or two.

Frankfort is the capital of Kentucky, in the United States. There are no castles in the vacinity, and I doubt that you can get to Europe and back in 15 hours.