The most important practical change to know about. Entry to the Pantheon used to cost nothing. It is now ticketed, at a modest price by Rome standards.

Several categories still enter free — residents of Rome, under-18s, people with disabilities and their companion, and worshippers attending Mass — and reduced rates apply to some age groups. Identification is checked.

Tickets can be bought online in advance, or on the day at machines and a desk by the entrance.

Buying on site is possible but the same-day queue can run from ten minutes to three quarters of an hour depending on the hour, and the on-site machines sell only for that day.

Book online if your schedule is tight. If you are flexible, an early arrival and a machine ticket works perfectly well and saves the booking fee.

Two queues exist and people confuse them: one to buy a ticket, and one to go in.

With a ticket bought online you skip the first and join the second, which moves quickly.

The queue is at its worst in the middle of the day and at weekends. Early morning and the last hour before closing are noticeably quieter.

There is no cover in the square, so a long wait in July is unpleasant and in the rain it is worse.

One detail worth knowing: the building closes to visitors for Mass and for religious services, and those times are posted. Turning up to a closed door because a service is running is a common frustration and is entirely avoidable by checking first.

What you have come to see, and it is worth knowing what you are looking at.

The dome is the largest unreinforced concrete dome in the world and has been for nearly two thousand years. Its diameter and its height to the oculus are identical, so a perfect sphere would fit exactly inside the space.

The concrete gets progressively lighter toward the top, with heavier aggregate at the base and volcanic pumice near the crown — one of the reasons it has survived.

The oculus at the centre is open to the sky. There is no glass. Rain falls into the building and drains through holes in the sloping marble floor, which are visible if you look down.

On a bright day a disc of sunlight moves across the interior through the course of the day. Standing under the oculus in the rain is a genuinely strange experience and worth timing if the weather offers it.

It is a working church, and has been since the seventh century, which is the reason it survived when so much of ancient Rome did not.

Raphael is buried here, in a tomb on the left as you circle the interior, along with two kings of unified Italy.

The original bronze doors are still in place and are the oldest of their kind in Rome.

The marble floor is largely original, and the coloured stone in the columns came from Egypt.

The interior is a single space. There are no side chapels to explore and no upper level, which is why the visit is short.

The Pantheon supplies almost no interpretation. You walk into a magnificent empty room and nothing tells you why it matters.

That is the difference between visitors who describe it as breathtaking and those who describe it as a large room they were in for ten minutes.

An audio guide is available and is worth it. Bring your own headphones — the interior is echoing and crowded, and listening on a speaker is unpleasant for everyone.

A guided tour covers more and puts the building in the context of the Roman Forum and the imperial city.

The cheapest option is twenty minutes of reading the night before. Know that Hadrian rebuilt it, that the dome is unreinforced concrete, and that it became a church — those three facts change the whole visit.

Twenty to thirty minutes inside is what most visitors report, and that is honest.

Add the queue, which can be anything from five minutes to three quarters of an hour.

An hour covers the visit comfortably with an audio guide and time to sit.

This is a short stop rather than an afternoon, which is why it works so well folded into a walking route through the centre — Piazza Navona is five minutes away, the Trevi Fountain ten, and Campo de’ Fiori a similar distance.

Do not build a half day around it. Build a walk around it.

First thing after opening is best: shortest queue, emptiest interior, and the light through the oculus at its most dramatic angle.

Mid-morning to late afternoon is the busiest, with tour groups and the ticket queue both at their peak.

The last hour before closing thins out again.

After closing, the exterior and the piazza in front are lit and are worth seeing in the evening — the portico columns floodlit, with the square much quieter than by day. That costs nothing.

A rainy day is an underrated choice. The rain falling through the oculus is the thing people remember, and the queue is shorter.

October and September are the busiest months here, with July close behind — Rome’s autumn shoulder is no longer a quiet season.

January is the quietest, then December and November.

Being indoors and short, this works in any weather, and the interior is a good refuge from both summer heat and winter rain.

Winter light through the oculus is lower and longer, which some find more atmospheric than the high summer sun.

February and November are the value months: short queues, mild weather, and the centre of Rome walkable without crowds.

It is a church, and the usual rules apply: shoulders and knees covered, hats off, no loud conversation.

Enforcement varies but staff do turn people away, and the shops around the square will happily sell you a cover-up at a price that reflects your predicament.

Silence is requested in the interior and is periodically announced. It is not always observed.

Photography without flash is permitted.

The building closes for Mass. If you want to attend rather than visit, that is free and is a considerably calmer way to be inside.

The Pantheon sits in the middle of the historic centre, and there is no metro station close to it — the centre of Rome is deliberately under-served by the underground because of what lies beneath it.

That means walking. From Piazza Venezia it is about ten minutes, from Piazza Navona five, from the Trevi Fountain ten.

Buses stop nearby on the larger streets.

Taxis cannot reach the square itself, which is pedestrianised, so expect to be dropped a short walk away.

The approach through the narrow streets, where the portico appears suddenly at the end of an alley, is one of the better arrivals in Rome. Walk in rather than being delivered.

The square in front has cafés with a view of the portico and prices to match. A block away in any direction the same coffee costs a fraction.

Two of Rome’s best-known coffee bars are within a two-minute walk and are worth the detour.

Sant’Ignazio, a few minutes away, has a painted false dome that is one of the great illusions in Roman art and is free.

Piazza Navona, Campo de’ Fiori, the Trevi Fountain and Largo di Torre Argentina are all within a ten-minute walk.

This is the densest part of Rome for sights, and the Pantheon is best used as the anchor of a half-day walk rather than a destination in itself.

Good for a two-thousand-year-old building. The interior is a single level and step-free from the portico, and the entrance has a ramp.

The piazza outside is level, though the surrounding streets are cobbled and uneven, which is the harder part.

Free admission generally applies to visitors with disabilities and a companion.

There is limited seating inside and none in the queue.

The crowd density in the middle of the day is a barrier in itself, which is one more argument for an early visit.

The complaints since ticketing began are predictable and mostly about the principle rather than the price.

The underwhelming version: arrive at midday without a ticket, queue in the sun for forty minutes, walk in, take a photograph of the oculus, walk out, and wonder why you paid.

The good version: read twenty minutes about Hadrian and the dome the night before, arrive at opening with a ticket bought online, stand under the oculus while it is quiet, walk the circle to Raphael’s tomb, and come out into the square for coffee a block away.

It is the best-preserved building of ancient Rome and the engineering is still not fully explained. Half an hour, done properly, is one of the best things in the city.