We studied the spin arrangement in ultrathin antiferromagnetic Mn films in contact with a ferromagnetic Fe(001) substrate using spin-polarized scanning tunneling microscopy. Mn shows a layerwise antiferromagnetic order on Fe(001). In regions where Mn overgrows Fe steps, a frustration of the antiferromagnetic order occurs which is similar to a 180^° domain wall. This topologically enforced frustration was studied as a function of Mn thickness. A linear increase of the width of the frustration region with the Mn thickness was found.