The Katyn Museum in Warsaw, Poland, one of the five finalists for the Mies van der Rohe Prize this year, shows just how much architecture you can make out of found space. The museum, completed in 2015, commemorates the victims of a massacre that occurred more than seven decades ago. Using a military fortification in the center of the city, far away from the three forest sites where the massacres took place, the museum brings these places and times into the present, while its exhibits tell the story in a manner you could find in book or documentary movie.

The history of the massacres involves one of the strangest episodes of the postwar era. In 1940, Soviet troops killed an estimated 20,000 political prisoners—Polish officers, policemen, members of the intelligentsia—that they were holding in camps in Russia, the Ukraine, and Belarus, burying them in mass graves. In 1943, Nazi occupation forces exhumed the graves in the Katyn Forest, outside of Smolensk, Russia, yet the Soviets managed to convince the Allies that the victims had been killed by the Germans and used the misinformation to rally Poles to their side. The question of who was actually responsible became a cause of much dispute during the Cold War. Only in 1990 did Mikhail Gorbachev release the files that included the order, personally endorsed by Joseph Stalin, to kill the victims.

The Polish government, in order to commemorate the misdeeds of their erstwhile allies, decided to do so in the middle of their capital, chosing a 19th-century fortress ironically built by the Russians when Warsaw was part of their empire. Local firm BBGK Architekci won the competition in 2014 by proposing to work with what was there, rather than create a brand-new structure.

As you approach the fortress, you get a hint of what awaits: a slice cuts through the glacis, or earthen redoubt, that contains the site, giving you a sense that both nature and defense have been interrupted, but without giving you a clear sense of why or how. You pass by the cut, through a gate, turn, walk by the postwar barracks the Polish army inserted here, and find yourself in a square in the fortress’ southern corner. A grove of trees sits in the center of this large but isolated space, through which a path zigzags its ways to the museum entrance proper. Through the reversal of the forest in the opening, rather than the opening in the forest where the graves were found, the space makes the far-away location present.

Inside the museum, the displays are more or less typical presentations of history, complete with introductory videos and panels that tell the stories of the larger event and of the lives of those who disappeared. The architects used the arched spaces of what had been the fortress storehouses to give weight to the displays and create a sense of enclosure and theatrically lit darkness. The best parts of the displays are gridded racks that hold small items, from amulets to homemade chess pieces, recovered from the graves at Katyn and two other sites, which humanizes those who lost their lives.

You exit from the museum into the narrow space between the glacis and the brick walls that separate the fortress from the surrounding town. You move through a tunnel, designed by the artist Jerzy Kalina, emerging into the off-kilter space between the earth slope and the towering perimeter wall. The architects planted a few trees, and peopled the sequence of spaces, divided by an existing series of cross walls, with stone steles on which they inscribed the names of the professions of those who were killed. To me, this was the museum’s most powerful space; there, you find yourself unbalanced, cut off from the world, with a sense of being overwhelmed, yet the spaces are also open to the sky and thus a larger, but abstract, universe.

Another found space stands at the end: a brick building BBGK divided into four chapels, one for each of the faiths of those who died (Roman Catholic, Eastern Rite, Jewish, and Muslim), although the names they had inscribed on metal panels do not follow those faiths. Here you can sit and remember the victims, in the manner of a traditional memorial. Then you exit, either down a hallway lit at its end by a skylight high above, where you can take an elevator back to the first square where you entered, or by climbing up the stairs in the cut you saw when you first came to the outside. The defensive structure of the glacis again overwhelms you, yet you are cutting through it, opening up a world of war, defeat, and death until you stand at the top, and look back, at which point you see not the memorial, but Warsaw itself: a car repair shop, a messy square, and blocks of apartment buildings that are signs of life as usual.

The origins of architecture lie as much in tombs and memorials as they do in habitation, and the Katyn Museum takes us back to that sense of the fixing in one place of lives that were lived, but are now gone. As a grave marker you come across in the forest, which was Adolf Loos’ definition of pure architecture, it is effective and clear, despite its remove from the actual location. It reminded me exactly of the alienating, removing, and making present that are at the core of monumental architecture, and I certainly hope the Mies van der Rohe Award jury recognizes its power when it announces the prize winner in May.