help needed in reading Walden

In the first chapter Economy Thoreau talked about shelters. He dug his own cellar in the side of a hill sloping, and then he went on to remark: “Under the most splendid house in the city is still to be found the cellar where they store their roots as of old, and long after the superstructure has disappeared posterity remark its dent in the earth.”
I don’t really understand “their roots as of old”. What are the roots? Why “of old”? Does it mean “they store some roots of vegetation in the way our ancestors did”?

I'm not a huge fan of Thoreau (he seems to have a much wider reading in the US of course) but I would interpret it as follows -

they stored roots (like potatoes, turnips, carrots and other root vegetables that keep longer in a dark place) the same way people used to in the past before the days of refrigerators.

Of course, metaphorically he is suggesting that much of the wisdom that was learnt in the past was buried over time, but it is still evident in the present if we know where to look.

Hope this helps you.