Logan Square, one of William Penn's original city parks, hasn't been a true square since the early 20th century, when the city began plowing the Ben Franklin Parkway through Center City's northwest quadrant. The heart of the square was turned into one of the world's most elegant traffic circles, home to Alexander Stirling Calder's Swann Fountain, while the remaining pieces were cast adrift, an archipelago of mournful traffic islands, uninhabited and bleak.

The Center City District has made a mission out of reclaiming those pieces, starting a few years ago on the west side with the scooped trapezoid called Aviator Park, opposite the Franklin Institute. Now, working its way east, it has just rescued the orphan outside the chocolate-colored Cathedral Basilica of SS. Peter and Paul, a space that goes by the name of Sister Cities Park. The transformation of this tiny shard of land amounts to a small miracle.

Designed by Bryan Hanes Studio and Digsau, two young Philadelphia firms that were practically start-ups when the project began, the bare-bones tribute to Philadelphia's 10 "sister cities" has been reconfigured into a multifunctional, multigenerational refuge. Sister Cities Park captures the refined whimsy of Paris' Luxembourg Gardens and packs it into a space a quarter the size of Rittenhouse Square. Yet the new park manages to feel cozy and open all at once.

It's hard to believe that such a small site — 1.75 acres — can accommodate this many activities. You can now climb a mountain in Hanes' delightful children's garden, launch toy sailboats on great adventures in the pond, dodge a gusher of water in the splash fountain, and then repair to Digsau's crisply elegant pavilion for lunch. (That is, if the cafe operator, Milk & Honey, doesn't run out of food, as it did the day I visited.) With all these options, there is still plenty of room left for a broad lawn, shaded by mature London Plane trees saved from the park's previous incarnation.

The $5.2 million overhaul is among several public improvements organized in advance of the Barnes Foundation's arrival at the northwest corner of Logan Square, and was paid for with grants from private donors and state agencies. Initially, it was seen as a way station for tourists making the desolate, 1.2-mile trek along the Parkway between City Hall and the Philadelphia Museum of Art.

Instead, the park has turned out to be something more important: a destination for the families who make their home in the city. An easy walk from the surrounding rowhouse neighborhoods of Logan Square and Fairmount, it is already drawing the local stroller set, as well as families who come into Center City to use the Free Library's main facility and attend Mass at the cathedral. If you don't have a small child of your own, try to borrow one for a visit.

Not that the new park is just for kids. Hanes' landscape and Digsau's architecture are designed to please discerning adults, too. Hanes, who developed the initial concept, envisioned Sister Cities as a super-concentrated version of Fairmount Park and the Wissahickon Valley. The idea is an obvious nod to the Parkway's original role, as a landscape aqueduct to funnel the country experience into the city, but he pulls it off without creating a theme park.

At the north end of the park, Hanes fashioned a miniature Wissahickon landscape. You climb a hill along a curving path, past schist boulders, fallen logs, and native plants such as black chokeberry. Along the way you encounter a narrow creek and seats resembling giant mushrooms. Occasional jets of mist, rising like morning dew, add to the alpine experience, although the true source is a concealed cistern of recycled water.

From the top of the hill, you get a telescoped view of Center City's towers pressing at the park's edge like a magical Oz. Despite the difference in scale, the buildings seem to cradle the park rather than dominate it.

The vantage also offers a clear understanding of the park's composition. Digsau's sophisticated glass-and-stone pavilion holds the center.

Just beyond, bracketing the building, Hanes created a new tribute to Philadelphia's Sister Cities, a circular splash fountain where concentric rings representing the different cities rotate outward. They are spaced in proportion to each city's distance from Philadelphia. The flat circle of the fountain, which can be turned off in cold weather, counterbalances the round topographical hill on the other side of the cafe. The arrangement happens to be a variation on the composition of JFK Plaza down the Parkway, where the circular fountain and spaceship orbit each other.

Here, Digsau's cafe offers a counterpoint of sharp corners. The architects kept the Wissahickon theme by cladding the building in a gray suit of Emerson limestone to evoke the local schist. It's cut in long bricks that are smooth in some places, roughly cleft in others, to reflect the tension between urban and rural embedded in the design.

Unlike the landscape, the pavilion's heart is with the city. Its roof tilts up at a jaunty angle, revealing a sharply creased glass corner. The glass is so clear, there is almost no distinction between the indoor seating and the section outside under the wood-clad awnings. You are part of the city as well as the countryside.

Hanes also introduced a diagonal walkway at the southeast corner that allows pedestrians arriving from the east to merge seamlessly onto the Parkway, without struggling with its oddly located crosswalks. The cafe also includes another passage connecting the park to the cathedral. An attached structure serves as a boat rental.

The only edge of the park that fails to connect fully with its surroundings is the northeast corner, at the base of the hill. Hanes felt a protective fence was necessary for safety. But the treatment of the corner effectively turns its back on the new Mormon Temple, which is being built at 18th and Vine Streets and will be an important addition to the Parkway's monuments. Despite all our technological innovations, we seem incapable of constructing urban projects that relate on all their sides to the surrounding city.

The freshness brought to Logan Square's disconnected pieces by the Center City District, and by funders including the William Penn Foundation and Pew Charitable Trusts, is as invigorating as a morning walk in the Wissahickon. These private donors have raised design standards enormously for Center City. Now, the question is how the same can be done for parks outside the tourist zone. Every Philadelphia neighborhood should have a park as good as Sister Cities.

Contact architecture critic Inga Saffron at 215-854-2213, isaffron@phillynews.com, or @ingasaffron.