Downtown? What downtown? We don’t have one.

If your talking about Assembly Row, that’s a “town center” development. Used to be a movie theater and a sea of parking lots. It doesn’t factor into Somerville’s style of urbanism.

You could argue Davis Square or Union Square I suppose. Bit, no, there's really no urban core the way Boston has one. I'd note that, in spite of that, Somerville is the 16th densest city in the US (with the caveat that comparing city densities involves somewhat arbitrary political boundaries). Cambridge is pretty much in the same boat and is at #26.

OP called out "skyscrapers". What counts as the precise cutoff for "tall buildings" is always going to be a matter of opinion.

But I live in Cambridge, MA and I can assure you Somerville would remain one of the densest cities in the union if every last building therein was lopped down to 3 stories by a giant lawnmower.