The Abbey of Saint-Père-en-Vallée was a Benedictine monastery just outside the old city walls, founded in the 7th century and rebuilt repeatedly over the medieval period. The Gothic church that survives — now the parish church of Saint-Pierre-en-Vallée — dates largely from the 13th to 15th centuries and has fine stained glass of its own, older than the cathedral's but less celebrated.
The church is worth visiting if you are already walking along the Eure, partly for the glass and partly for the carved portal. The surrounding neighbourhood, with its half-timbered houses and views up to the cathedral, is among the most photogenic in Chartres.