Thanks for sharing this, I've never seen it before and loved it. And I was curious, so I did some research:

What is the Arch of Constantine?

The Romans built arches like this as a sign of respect for a conquering army commander. They built them along the route the army would take when it came home - think of the V-Day celebration in NYC with the guy kissing the girl - an equivalent would be Congress building an arch in Times Square to celebrate Roosevelt. Or rather, an existing arch in Times Square would be reformatted to show Roosevelt's face, rather than Washington's.

Similarly, the Senate voted to replace the figures of older victorious leaders with Constantine's - and really only his face. Notice the faces are gone as of 1745:

Constantine vs Maxentius

It's kinda a long story, but Constantine was named emperor while in England. His rule was briefly usurped by Maxentius. During this time, Maxentius rebuilt much of Rome. Constantine found out and rolled into Rome, killing Maxentius. The Senate, in order to keep the peace, remade the arch in the image of Constantine, pretending it was always him they were in support of. Later he rolled out, back to England and didn't return to Rome until many years later.

If you ask me, maybe there shoulda been another emperor to look after the state of Rome, although by that time, much of the power had moved out of the city. Anyway, that's why it's called the Arch of Constantine.

The Painting

Canaletto was a tourist painter. Think of the people who will paint you as a caricature on the sidewalk in tourist hotspots. Similarly, Canaletto would paint pictures of places. He usually painted Venice, as that is where he operated, but in the 1740s he briefly dabbled in other places. This is one of those artistic excursions. Tourists would buy the paintings as a keepsake of their travels.

The Statement

Notice the figures in front of the arch.

The man with a cane in the lower-right:

The Woman sitting, leaning against the arch:

The man looking at the man carrying stuff, looking at the woman:

Notice the general disrepair of the arch and colosseum, mainly the greenery growing:

These are statements of societal class and the state of Rome. The man to the left, dressed in fancy clothing is rich and annoyed that his servant has stopped to talk to the woman on the ground. The man with the cane ignored by everyone.

It's interesting to see the wealth disparity outside of our own current context. This is 30 years before the creation of the United States, and 200 years before she started forcing her form of capitalism on the world. It's also interesting to note that through most of human history, cities were where people didn't want to live. They were run-down, stinky and dangerous places.

Now, they're the opposite. They attract the youthful or wealthy, willing to live in destitute or dish out money to be close to the action - although more recently, the balance has tilted to the latter, where the poor live in close vicinity to the wealthy, and more are pushed out every day in desire of larger tax revenues.

This is similar to Rome at the time of the arch - 315. The city was only for the wealthy. The servants lived outside of town or in retched squalor.

The Now

Here's what the arch looks like from the same view today:

You can't even see the Colosseum any more for the apartment buildings built between. There are high price tags to live among the views of ancient relics, and this is another statement that can be gleened. However, it should be noted the artist moved the colosseum to the left for the sake of composition. I'm not sure how far.