Marienkirche is Frankfurt's great medieval survivor, a large Brick Gothic hall church begun soon after the city was founded. It was badly damaged in 1945 and has been rebuilt in stages, which gives the interior a spare, slightly raw feeling.
The church is best known for its medieval stained-glass windows, including unusual scenes from the life of the Antichrist. Even if the glass is not the reason you came, the scale of the building explains why Frankfurt mattered as an Oder trading city.