Gauzy curtains blow in the breeze, framing a perfect view of the sparkling Mediterranean. The sun’s rays fall across our big white bed as we stand at the tall window, gazing down at a rocky shore, and right to a wide curve of beach. We could be in a five-star hotel. The setting, a 14th-century castle of pinkish stone overlooking golden sands 50km up the coast from Rome, is gorgeous enough to catch the beady eye of some property developer, but we’re actually staying in a new hostel, opened last summer in Castello di Santa Severa by the Lazio regional government.

In proper hostelling style, we arrive on foot, walking the 15 minutes from Santa Severa’s tiny station, a ride of under an hour (€4.40 one-way) from Rome’s Termini station. We soon see that it is more than just a castle; it’s an entire, walled medieval village, with houses, workshops, courtyards, two churches and sea on three sides.

It was owned over the centuries by various noble Roman families and the Catholic church (it was a summer bolthole for medieval popes), but lay neglected from the late 1600s, apart from a period in 1943 when the German army used it.

After four years of repair and restoration, the complex opened as a visitor attraction in 2017, with three museums (combined ticket €8/€6), an 11th-century tower to climb, and lots of events and exhibitions. It is popular with families and school groups, pulling in 50,000 visitors in its first year. A couple of potters plus jewellery and leatherware makers have opened shops in other old buildings, with more artisans to come. The hostel was added last summer.

Facebook Twitter Pinterest Castello di Santa Severa is a walled medieval village, with houses, workshops and courtyards

Italian politics seems to be all about the rise of the right, but the heirs of Eurocommunism are still active in areas of the south and centre. This castle is one of several “cultural lungs” preserved for the public by left-leaning regional president Nicola Zingaretti since his election in 2013.

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As it’s a Sunday, there’s a sprinkling of visitors, but we push on past them, under ancient archways to the oldest part of the complex, where 14 rooms and dorms sleep 42 in all. Our double has a wooden floor, rough-plastered grey and white walls and exposed roof timbers, nice linen and that lovely outlook (eight rooms have sea views). In the en suite, there’s a hair dryer, plenty of towels (no soap, oddly) and the window is a glazed arrow-slit in a wall over a metre thick.

Facebook Twitter Pinterest Private rooms have en suites and sea views

So far so boutique hotel, except there’ll be no pastries and fresh fruit laid out on a veranda next morning. Hostellers sort their own breakfast, but get to do it in a surprisingly high-end communal kitchen that lies over a courtyard (past the remains of an early Christian church). It has a beamed ceiling, TV, cute sofas and vending machines for coffee, snacks and drinks, as well as fridge and cooker.

After a cheap lunch in one of two friendly bistros just outside the castle walls (grilled aubergine and mozzarella roll €4), I’m keen to hit the beach, but first we check out the culture: a museum of the sea with lots of reconstructions of old ships – their holds packed with amphorae; a gallery of classical terracotta painting – on vases, tiles, floors; and a hands-on museum of mining and farming, presided over by genial retired chemist Giulio, born in the castle (in a wheelbarrow, he jokes) to kaolin miner parents.

The galleries are all well done – if you speak and read Italian. The hostel claims to be extending a welcome to youth from across Europe, so a next step needs to be more signage in English – as useful for Swedes and Slovaks as monoglot Brits.

Facebook Twitter Pinterest Santa Severa beach. Photograph: Liz Boulter

For an Italian public beach, the sands immediately north of the castle walls are delightful, wide and golden, with a few bars and the Isola del Pescatore fish restaurant. We stroll along the water’s edge, watching fishermen hoping to catch sea bass and bream, and a wetsuited class from the local surf school (scuoladisurf.com) catching a few waves.

Leafy Santa Severa village was created in the 1930s by Mussolini as a resort for Fascist cronies (but we don’t hold that against it 80 years on). Its square is lined with bars and gelaterias, and there are two bakery/delis and a small supermarket for breakfast and picnic supplies.

Facebook Twitter Pinterest The hostel’s communal lounge and kitchen

Mondays are the best day here: the castle is closed to day trippers, so hostel guests get the whole complex to themselves. We lounge with a picnic on scruffy grass overlooking the southern shore – the whole place is endearingly unmanicured. Later, on the beach, we discuss choices for the following days: a cycle hire shop does guided mountain bike excursions into the Monti della Tolfa, a few miles inland; there’s a riding stable behind the castle; and there are amazing views and wildlife at the Macchiatonda nature reserve a few kilometres south.

We’re in danger of forgetting that a major reason for coming to this “castle kissed by the sea” is the chance to combine seeing the sights of the Eternal City with an affordable beach break. The Vatican and the popular Trastevere district are on Rome’s west side, so they’re even nearer. Maybe tomorrow. Or the day after…

• Accommodation was provided by Castello di Santa Severa (dorm beds from €25, doubles from €65, castellodisantasevera.it). Flights were provided by Ryanair, which flies to Rome Ciampino from five UK airports. There is no ticket office or machine at Santa Severa station: tickets bought on board cost more, so buy returns in Rome, or via the trainline app

• This article was amended on 16 April 2019 to clarify that while train tickets can be bought on the Trenitalia app, you need to be an Italian resident to use it. Non-Italians can buy tickets to Santa Severa via the trainline app.