As a city, we're acutely aware of each discrete change to our physical surroundings. A new building rises here, a favorite haunt goes dark and vacant over there. Philadelphia, like all cities, changes little by little. So why, then, do the big, historic transformations so often take us by surprise?

Those were my thoughts as I stood at 12th and Catharine Streets, gazing across the impeccable lawn of the new Hawthorne Park, at the tidy rowhouses that now cluster so tightly at its edges. Adults sought shade under spindly new trees; children darted toward the curvy tangerine chaise longues arranged in the grass.

That scene, on opening day last month, was as sweet and placid as a New England commons, even with the glass spires of Center City poking over the rooftops. Yet only 13 years earlier I had waited in the very same spot with a few hundred others, tensed for the boom of dynamite. We watched the four gloomy towers of Martin Luther King Jr. Plaza turn to dust, one year shy of the housing project's 40th birthday.

Only now, with the completion of Hawthorne Park, designed by LRSLA Studio, a Philadelphia landscape architecture firm, is it clear how dramatically conditions have shifted in that once notorious, crime-ridden neighborhood just below South Street. Unless you knew what these blocks were like when the towers stood, you wouldn't find the slightest clue today, so thoroughly have the traces of turbulence been cleared away.

A happy ending was by no means a given, says Patricia Bullard, an activist with the Hawthorne Empowerment Coalition. "The whole area looked like hell after the towers came down, filled with abandoned buildings," she recalls.

It took several years before traditional-style brick rowhouses, designed by Torti Gallas & Partners, were built under the federal government's Hope VI program to replace a portion of the lost public-housing units. A few more years had to pass before the Philadelphia Housing Authority finished a promised cluster of market-rate houses, intended to diversify the neighborhood economically.

Then, boom: private development took off. Builders scooped up the empty lots that were a side effect of the towers' blight and inserted new homes, some quite grand. The old Hawthorne School became condos. Dranoff Properties finished a large, upscale apartment house, 777, at Broad and Catharine in 2010. Hawthorne Park completes the tableau of a thriving urban neighborhood.

The design, which was done for a modest $2.1 million, isn't as snazzy as the one for Sister Cities Park, which opened in the spring on Logan Square. There was no money here for jetting fountains or a sleek cafe. But LRSLA organized the little oasis - three-quarters of an acre - to suit the neighborhood, both its past and its present.

Since Hawthorne Park happens to occupy the site of the towers' old plaza, LRSLA designer Brad Thornton honored that history by creating a hard surface at the southeast corner. Paved in a distinctive orange-and-brown brick, the new plaza slopes up from the lawn to form a seating area that also multitasks as a stage for outdoor movies, farmer's markets, and other events. Granite-lined paths reach out at the corners to new neighborhood anchors, such as the Academy at Palumbo, the well-regarded public high school that was founded in 2006.

The rest of the park is pretty much lawn, where people can play impromptu sports, picnic, or sunbathe. The park, which adheres to landscape architecture's rules for sustainability, was designed to soak up as much rain as the skies can deliver, to keep the water from burdening the city's overtaxed water mains. Like all good parks, the open expanse of greenery helps make surrounding housing look more regal.

While the park design may be low on frills, it ties together beautifully with the surrounding neighborhood. The elevated plaza looks out at the spot where the towers' community center once stood - and where the Rev. Dr. Martin Luther King Jr. famously spoke to residents in 1965. The odd, stainless-steel lectern at the edge of the new plaza is actually an art piece memorializing the event, Object of Expression by sculptor Warren Holzman. One can imagine children and politicians alike declaiming there, something like Speaker's Corner in London's Hyde Park.

What would King say if he could come back and take the lectern? What would he make of the neighborhood now?

The revival of Hawthorne raises the usual, difficult questions about gentrification. The new, low-rise public housing is better built and, so far, better maintained than the towers that opened in 1960. Unlike the overpowering high-rises, the brick-fronted houses look like they belong in the neighborhood. But there are fewer affordable units now, roughly half the 576 apartments that existed in the four towers.

It's hard to imagine today that a neighborhood within an easy walk of City Hall, with excellent transit connections, was ever allowed to became a dangerous, no-go enclave. Yet, as the story of its resurgence sadly suggests, gentrification is one of the few effective tools that American cities have found to repair such damaged places.

The urbanist Jane Jacobs, in her landmark book The Death and Life of Great American Cities, predicted that left to their own devices, derelict parts of cities would eventually "unslum" with the same residents. As nice as it is to see Hawthorne's new development and vitality, that's not what is happening here - or in other reviving Philadelphia neighborhoods.

It's remarkable that this dizzying series of experiments in Hawthorne - clear-cutting a neighborhood, building a utopia of towers, rejecting the model and imploding the towers, then starting afresh with the original rowhouse form - all happened in the brief span of 52 years. The alien towers that Jacobs hated are gone, but her vision of a naturally unslummed neighborhood remains an elusive dream.