Nothing highlights downtown Boston’s turnaround from has been to gotta be there as much as the Millennium Tower/Burnham Building complex. The high-rise is now the city’s tallest residential building. But even more important, symbolically, it covers a huge chasm excavated by a Vornado-led development team that halted construction of a big complex during the recession.

Millennium Partners hit major milestones with yesterday’s topping off of its $700M mixed-use tower. For the city, it dramatically marks the resurgence of downtown. For the developer, it underscores the $1.2B in residential sales it’s made in two complexes in the last three years, partner Richard Baumert tells us.

The demand by companies and households to be downtown has been gaining momentum and led to the “staggering” sale of 90% of the 442 Millennium Tower luxury condominiums a year before first occupancy next summer and the leasing of all its office and most of its retail space, Richard says. That’s a nice return on the $1.5B the company has invested to develop three projects within blocks of each other downtown: The Ritz-Carlton Boston Common, Millennium Place and the latest tower.

One of the most daunting challenges was the gut renovation of the 1912 Burnham Building, Daniel Burnham’s only Boston project that for decades housed Filene’s. Today’s energy code is a world apart from what was required 103 years ago, Richard says. (Back then, most energy was harnessed from grunts released when William Howard Taft entered a bathtub.) They had to reconfigure the exterior of the north facade, which was obscured by another building for over a century. Kathy MacNeil, a partner in the development firm, managed the project built by Suffolk Construction.