This is the building that marked a turning point in 20th-century architecture?

Its walls are made from concrete block the color of wet cardboard, and the mortar that holds them together seems to have been squeezed straight from a tube. You won't see a single window when you arrive at the Trenton Bath House, never mind a conventional front door.

The New Jersey summer camp that commissioned this little pool house from Philadelphia's Louis Kahn didn't think much of the results. Kahn finished the building in the scorching summer of 1955, and was immediately fired from the project. Then, the European and Japanese tourists started showing up.

The spartan pool building that Kahn created for the Trenton Jewish Community Center isn't the sort of architecture you fall in love with at first sight. It's too plain and too cerebral, especially in an age when judgments are often made from photographic eye candy. The Trenton Bath House is a building that hides its wisdom in cool, shadowy corners, and is best experienced in person.

Soon, more people will have the opportunity to see it for themselves. Mercer County is wrapping up a painstaking $2.1 million restoration, the kind of preservation effort normally reserved for colonial-era cabins and majestic estate houses.

When its architects, Farewell Mills Gatsch, complete their work this summer, the Trenton Bath House, which is actually located just over the city line in Ewing Township, N.J., should look as pristine as the day it opened - maybe better. Of course, that doesn't guarantee people will ever warm to the solid concrete block walls.

You never know. Kahn's work began to come back in favor in the 1990s, and his reputation experienced a big bump after the documentary My Architect by his son, Nathaniel Kahn, was released in 2003. While the Philadelphia architect didn't complete a huge body of work during his career, ended by a fatal heart attack at age 73 in 1974, his major buildings have, one by one, been rediscovered and restored.

Now it is the turn of the Trenton Bath House, which is where architecture really began for Kahn, says Susan G. Solomon, a historian who has written extensively about the project. He was almost 50, and had just started his own firm, when he was hired by the Jewish group to design a day camp in suburbanizing Ewing Township. Even though he later lost the commission, the bath house - a rather grand name for the pool's dressing rooms - "was where he found himself as an architect," Solomon believes.

Kahn had been growing dissatisfied with conventional modernist design, especially the corporatized International Style and its emphasis on enclosing buildings in lightweight, nearly invisible sheaths of glass.

A visit to Gordon Bunshaft's Lever House, a crisp wafer of glass and steel on Manhattan's Park Avenue, left Kahn especially worked up. "It is dead," he wrote to Anne Tyng, an architect who would collaborate with Kahn on the camp project and become his mistress.

Kahn considered himself as much a modernist as Bunshaft, but he felt his clinical modern architecture had lost its human connection. Looking for a new approach, Kahn turned to classical antiquity, and sought to merge its primal essence and geometric forms with a modern sensibility.

The Trenton Bath House was Kahn's rebuttal to Lever House. Unable to afford real stone, he chose concrete blocks ground from Delaware River rock, so the building would sit heavy on the earth and you would feel the massiveness of its walls. Close up, you can see the rough block was an inspired choice, giving the modern pool house the dignity of a Levant ruin. The high, solid walls shield a serene refuge. Entering from the recessed side door is like navigating an ancient souk.

The bath house reads like a treatise of Kahn's ideas.

It's no accident that the floor plan is identical to those that Palladio used for his Italian country houses: a Greek cross composed of five equal-sized cubes. Only here the building's heart is open to the sky instead of being topped by a dome. In its place, Kahn incised a circle in the courtyard floor.

There are three real rooms: one changing room for women, one for men, and another room for storing street clothes. The fourth cube is a covered porch that elevates the walk to the pool into a grand procession. The four outer cubes are topped with that great modernist no-no, pitched roofs.

Instead of sealing the interior from the elements, the roofs hover several feet above the block, allowing air to circulate in the changing rooms and providing glimpses of the changing sky.

"No matter how hot it was, the air moved and it never smelled," marveled Donna M. Lewis, the county official overseeing the restoration, and a longtime member of the pool. The rooms have plenty of natural light, but it's never harsh.

Those rooms include the most celebrated Kahn feature: hollow pillars at the corners of the cubes. Some double as baffled entrances to the dressing rooms. Others enclose the toilets or closets. Kahn called them "servant spaces" because they support the functioning of the "served spaces" - the main rooms. Once Kahn discovered the advantages of such a hierarchy of spaces, he would apply the concept to all his major designs.

After the bath house became widely known, other architects joined Kahn in questioning modernist orthodoxies. That didn't make its owners like the bath house any more, recalled preservation architect Michael Mills, who first noticed its decay and raised a red flag. Thanks to Lewis' foresight, the county recognized the treasure in its midst and raised the money to turn the day camp on Lower Ferry Road into a public pool.

While the bath house's design is childlike in its simplicity, renovating it was not. Wu & Associates, the Cherry Hill preservation contractor that worked with Mills, tested six formulas before finding a perfect match for the concrete block. Masons had to be taught how to apply the mortar in the slapdash way Kahn liked.

The architects also contributed one of the missing pieces that Kahn didn't get to design properly: a snack bar. Similarly modest, its walls are built from concrete block, but of a newer vintage. Its cedar roof flutters up like a butterfly wing, in contrast to Kahn's downward pitch.

A few other architects still take their cues from Kahn's modest pool house, but they're swimming against the tide in our bling-obsessed times. Look at the grief that Tod Williams and Billie Tsien experienced after the release of their understated design for the Barnes Foundation, another thoughtful arrangement of boxes with a secreted entrance. The return of Kahn's humble relic comes in the nick of time.