When in Ginestar the best way and most interesting way to get to the castle in Miravet is to cross the Ebro river by the ferry.
Basically the ferry is run on the power of the river itself. No motor involved.
The ferry can take around two cars and a few bikes but it was only our car for our trip.
As far as i'm aware, the ferry has been running for over 50 years. The ferry operator showed us pictures in black and white of two Fiat 500s on the ferry itself. It seems that Spain was full of Fiats years ago but they have all gone or dont work.
You get some fantastic views of the castle whilst you are crossing the river so get the camera ready.
You will also be so surprised how fast the ferry crosses the river especially when you are nearing the middle.
I strongly recommend this trip. You will enjoy it.