Dear maester Steven, for what reasons do you think Barrowton has been able to survive for thousands of years and why do you think it has never expanded into a city?

Barrowton makes sense to me. In addition to whatever religious significance the barrows once had that once might have brought pilgrims, it’s located on a hill at the confluence of two rivers, which makes it ideal for defense. There also aren’t any other centers to rival it on the southwest, and I think Torrhen’s Square is much more recent (probably having been built as a military outpost against the Ironborn). 

The Barrowlands and the Rills, being some of the more southerly regions of the North, probably are more suited for agriculture than much of the North, so they probably produce quite a bit of the region’s cereal grains. I also think, based largely on geography and the sigil of House Ryswell, that the Rills are the center of horse-breeding in the North, so Barrowton would be the logical place for a horse-fair. 

As to why it never expanded into a city – I think the primary culprit is the North’s decision to destroy its naval capacity on the west coast under Brandon the Burner (one of the great mistakes in Northern history, IMO). Without a strong naval presence in the Sunset Sea, the presence of the Ironborn just over the other side of the Flint Cliffs meant that trade with the Riverlands and points south was safer to do via land (and with their seat at Moat Cailin to allow them to tax land trade, I think the Starks concurred.), or by shipping through White Harbor instead, which was protected from the Ironborn by a whole continent.