I was standing in a cave Picasso once slept in. Actually, it’s more of a ledge beneath a craggy, overhanging limestone rock. Yet, in the summer of 1898 the 16-year-old Picasso and a friend spent a month sleeping here: the Santa Bárbara mountain and the the wider Els Ports area in the Terres de l’Ebre region of Catalonia, some 210km south-west of Barcelona. He would later write: “Everything I know, I learned in Horta.”

Local lore has turned the ledge into a cave but, either way, it’s certainly remote – high up amid the dense holly oak and pine forests of the mountains. Apart from the odd derelict farmhouse, there was nothing for miles and all I heard were warbling sparrows and the trickling of a brook.

The answer to why Picasso came here lies with his friend Manuel Pallarès, a fellow pupil at the School of Fine Arts in Barcelona, who had invited him to spend the summer at his family home in the hilltop village of Horta (now Horta de Sant Joan). The boys decided to escape the summer heat by heading to the mountains and they spent August drawing and painting the landscape while Pallarès’s sister would bring provisions on a donkey every few days.

Several of the works Picasso created in Horta will be on show at Picasso and Paper, a new exhibition at London’s Royal Academy, which opens on 25 January. Among them will be a prodigiously accomplished drawing of a local village girl. There are also some of Picasso’s early forays into cubism, which he began experimenting with on his second trip to Horta 11 years later, accompanied by his then lover Fernande Olivier.

To a city boy like Picasso, raised in Málaga, A Coruña and Barcelona, the countryside in one of the furthest flung corners of Catalonia must have seemed like the end of the Earth. As I drove from Barcelona, the motorway cut through the dramatic limestone gorges of the Serra de Llaberia mountains, with no villages in sight for over an hour and a half. Eventually, a steep, winding, mountain road edged its way past vineyards, olive groves, almond trees and rust-coloured rocks to the walled, medieval village of Horta de Sant Joan.

Despite narrow streets, pretty, cloistered plazas and a slow pace to life, this is not an area much visited by tourists and walking into the Bar La Bassa on the square I got a few stranger-in-town stares from the elderly gents playing cards inside. Yet Horta is now part of a Picasso Route taking in the main places he stayed in and visited – in the village itself, as well as the nearby mountains. The starting point is the Centre Picasso museum, where I saw copies of the drawings and paintings Picasso created in the area, including wonderful sketches of village women washing their laundry in the streams and his cubist views of the Santa Bárbara mountain, memorably merged with the face of Olivier.

On the Plaza de Misa, the facade of the pension where Picasso and Olivier stayed was unchanged. The story goes that the deeply traditional villagers were so outraged by the idea of an unmarried couple openly sharing a room that they began throwing stones at their window. Picasso apparently responded by storming onto the balcony and firing a pistol into the air. There’s also a mirador (viewing point) on the edge of the village that provided me a head-on view of the Santa Bárbara mountain, recognisable from Picasso’s paintings, despite his cubist deconstruction.

To me, what was most impressive was the rural landscape of the Els Ports mountains in which Santa Bárbara sits – now a natural park. This is fantastic countryside for walking, cycling or a leisurely drive and, as well as the “cave”, it’s also possible to visit sites that Picasso painted, including the medieval Convent de Sant Salvador, with its beautiful Renaissance cloister.

The Picasso Route can be explored in a day but there are plenty of other things to see in the area. The town of Gandesa – about 30 minutes’ drive north-east – has a wine-making cooperative in a vast, turreted art nouveau “wine cathedral” and tours can booked of its interior. There’s also the impressive Centre d’Estudis de la Batalla de l’Ebre, a museum dedicated to one of the defining events of the Spanish civil war – the Battle of the Ebro – with interactive displays and powerful filmed interviews with survivors.

In light of Franco’s victory in the war, Picasso never returned to Spain, vowing that he would not do so until democracy was restored. But, for the rest of his life, a photograph of Santa Bárbara mountain took pride of place in each of his studios. The Catalan writer and Picasso biographer Josep Palau i Fabre believed that “[For Picasso], Horta represented the Lost Paradise, that lost paradise that almost everybody, in one way or another, carries inside.”

Horta is the kind of place which, if it were better-known, would be overrun with coach parties. Mercifully, it’s not and, on a clear day, as I looked out to the Santa Bárbara mountain, I got a sense of the paradise Picasso once found.

The trip was assisted by Catalunya and Terres de L’Ebre tourist boards. Near Horta de Sant Joan, Hotel Can Josep (doubles from €72.25 B&B) has panoramic views over the countryside

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• This article was amended on 20 February 2020 to clarify that Picasso and his friend spent a month sleeping in the wider Els Ports area, not just at the Santa Bárbara mountain as an earlier version suggested.