One of the most comfortable ways to travel is by plane and to come to Rototom you can land at the airports of Castellón, Valencia or Barcelona.
From the airports of Barcelona and Valencia you can use the bus transfer services of the company Mediterráneo Holidays, which will take you directly to the festival site from the airport. You must buy tickets online in advance, as it is not possible to buy them on the bus itself. Info and reservations: https://mediterraneoholidays.com/festivales/
On the other hand, if you arrive at Castellón airport you will have to take a bus to Castellón station or to Torreblanca train station. Once in one of these two stations you will have to take a train to Benicàssim. More information: https://aeroportcastello.com/es/aeropuerto/acceso-y-transporte/