The Maginot Rhine defenses employed three lines of defense, with blockhouses or casemates close to the Rhine (the first line), backed by infantry shelters (the second line). The third line was a strong series of casemates, built on the model of interval casemates in the northeastern sections of the Line, but without lower levels. All of the Colmar fortifications were built in the mid-1930s by the Commission d'Organisation des Régions Fortifiées (CORF). CORF was responsible for the major fortifications of the Maginot Line, but in this area no major positions or ouvrages were built.