The long oval form is not a design choice. The square sits directly on the footprint of the Stadium of Domitian, a first-century arena for athletic contests, and the buildings around the edge follow the line of the old seating.

That is why it is narrow, curved at one end, and far longer than a typical piazza.

Parts of the stadium survive beneath the square and can be visited — there is a small underground site with an entrance at the northern end, ticketed separately and rarely busy.

Above ground, the square as you see it is seventeenth-century baroque, laid out under a pope whose family palace occupies one side.

It is free, open at all hours, and entirely pedestrian.

Three fountains run down the length of the square and the middle one is the reason people stop.

The Fountain of the Four Rivers by Bernini is the centrepiece: four colossal figures representing rivers of four continents, arranged around a rock supporting an Egyptian obelisk, with the whole mass appearing to balance on nothing.

Walk all the way round it. The figures are designed to be read from different angles and the composition changes completely as you move.

At the southern end is the Fountain of the Moor, and at the northern end the Fountain of Neptune, both older and both worth a look.

The church facing the central fountain, Sant’Agnese in Agone, is by Borromini and is free to enter.

The one warning about this square, and it is consistent in visitor accounts.

The restaurants around the perimeter have the best position in Rome and charge for it. Quality is generally mediocre, portions modest, and cover charges and service add up. Menus in six languages and a man outside inviting you in are the reliable signals.

That does not mean avoid them entirely. A coffee or a drink at an outdoor table, paying tourist prices for a seat in a beautiful square for an hour, is a defensible transaction and plenty of people are glad they did it.

Eating a full meal there is the part to think twice about.

Walk two or three streets in any direction — toward Campo de’ Fiori or the streets behind the square — and the food improves sharply while the price falls.

The square is full of portrait artists, caricaturists, painters selling views of Rome, and buskers.

It is a long-standing part of the character rather than a recent nuisance, and the standard is often high.

If you sit for a portrait, agree the price before they start. This is where disputes happen.

Street performers work the central area, particularly in the evening.

The usual sellers of light-up toys and selfie sticks operate here too, along with the bracelet approach and the petition routine, both of which are best met with a firm no and continued walking.

Early morning is the square at its most beautiful and least populated — the fountains running, the light on the ochre facades, and almost nobody there. Worth setting an alarm for once.

Mid-morning to late afternoon is busy but pleasant, and this is when the artists are working.

Evening is the classic time: the fountains lit, the tables full, the noise level up, and the square at its most alive.

Late at night it empties again and the lit fountains with a handful of people around them are as good as the early morning version.

Unlike almost everything else in Rome, this square is enjoyable at any hour — the difference is the character rather than the quality.

May, October and September are the busiest, with June and July close behind.

March, January and February are the quietest.

The square depends on being outdoors, so it is at its best in the warmer months when the tables are out and the evenings are long.

Winter has a specific attraction: a Christmas market fills the square through December and into early January, with stalls, a carousel and a very different atmosphere.

April and October are the balanced choices — warm enough to sit outside, and without the summer crush.

The square sits in the densest part of Rome for sights, and everything is walkable.

The Pantheon is five minutes east. Campo de’ Fiori, with its morning market, is five minutes south. The Trevi Fountain is ten to fifteen minutes. Castel Sant’Angelo and the river are ten minutes west, with the Vatican beyond.

Two churches within a couple of minutes hold major Caravaggio paintings and are free to enter — San Luigi dei Francesi and Sant’Agostino. These are among the best free things in Rome and are missed by most visitors.

The streets between Navona and the river are full of small workshops, bookshops and bars, and are worth a wander in their own right.

There is no metro station near Piazza Navona — the centre of Rome is deliberately under-served by the underground because of what lies beneath it.

Walking is the answer. From the Pantheon five minutes, from Campo de’ Fiori five, from Piazza Venezia fifteen.

Buses stop on Corso del Rinascimento and Corso Vittorio Emanuele, both a couple of minutes away.

Taxis cannot enter the square, which is pedestrianised, so expect to be dropped on a surrounding street.

The approach through the narrow lanes, where the square opens up suddenly, is part of the pleasure. Walk in rather than being delivered to the edge.

An easily missed extra. The remains of the Stadium of Domitian are open to visitors, entered from the northern end of the square and descending several metres below street level.

It is a small site, ticketed separately, and rarely crowded.

What you see is the substructure of the stadium’s seating and entrance arches, which makes the shape of the square above suddenly obvious.

Allow half an hour.

For anyone interested in how Rome is built on itself, this is one of the clearest demonstrations available and it costs a fraction of the major sites.

The square itself is flat and largely open, which makes it one of the easier central Rome sights for wheelchairs and strollers.

The surface is cobbled with the large irregular Roman setts, so it is bumpy rather than difficult.

The surrounding streets are narrow, cobbled and often without pavements, which is the harder part of getting here.

The underground stadium site involves steps and is not accessible.

Sant’Agnese in Agone has steps at the entrance. There are no public toilets in the square; cafés expect a purchase.

Twenty minutes to walk the square, look at the three fountains properly and put your head into the church.

An hour if you are sitting down for a coffee and watching the artists.

An evening if you are eating nearby and coming back afterwards for the lit fountains.

Add half an hour for the underground stadium, and fifteen minutes each for the two Caravaggio churches a couple of streets away.

It belongs in a walking route rather than as a destination, and the natural loop is Navona, the Caravaggio churches, the Pantheon and back.

There is very little criticism of this square beyond the restaurant prices, and that is easily managed.

The mediocre version: arrive at eight in the evening, sit at the first table with a menu in five languages, pay a great deal for average pasta, and leave with an expensive memory of a lovely view.

The good version: walk in early one morning when it is empty and the fountains are running, come back in the evening for the lighting, have a drink at an outdoor table if you want the seat, and eat two streets away where the food is better and half the price.

It is free, it never closes, and Bernini’s fountain is one of the great things in Rome. The only decision that matters is where you eat.