Do all roads lead to Rome?

The ancient road network built by Romans played an important role in their expansionist policy. Thanks to that, it was possible to move rapidly legions rapidly through the vast Roman Empire.

The well known expression that all roads lead to Rome is partially true. Actually, what is really true is that Rome was born (was created) at the place where two important ancient trade routes were intersected.

They are were trade routes: the inland route to get salt to from the sea (and other goods?) and the river route that Greek sailors used during the summer to sell their goods.

That crossing (or intersection) was an the important ancient market place that slowly became the eternal city of Rome.

It is considered best, whenever possible, not to begin a sentence with a conjunction (but, and, etc). The semicolon (:) can be used after a complete sentence to give a list.

"...is partially true." is correct, but more often said as "...only partially true"

Since you added a specification to "an important market place", I changed it to "the important..."

Generally speaking, I will say you write very well. :)

Do all roads lead to Rome?

The ancient road network built by Romans played an important role in their expansionist policy, thanks to that it was possible it enabled them to rapidly move rapidly legions through the vast Roman Empire.

But the well known expression that all roads lead to Rome is only partially true. Actually, what is really true is that In fact, Rome was born in the place where two important ancient roads routes were intersected: the salt road and the river Tiber. (Routes because a river isn't a road. Listing them here makes the next sentence easier to construct.)

They are the road of salt, the way that people coming from inland cities used to travel to get some salt to the sea, and the river that runs through the city, the sacred blond Tiber, that Greek sailors used to go up during the summer to sell their goods.

(It is hard to construct a long sentence like this correctly. It is much easier to break your points up into smaller sentences, especially when you signpost them beforehand.)

People from inland cities travelled along the salt road to get salt from the sea. Greek sailors used to travel up the sacred blond Tiber during the summer to sell their goods.

That crossing This intersection was an important ancient market place that slowly became the eternal city of Rome.