At the fortified hilltop settlement of Tonovcov grad near Kobarid in western Slovenia, window glass was found in the layers of use and destruction of a late antique house and church complex. Two natural colour types (yellowish and green) are represented. The glass shows the usual characteristics of the cylinder-blowing process; some of the panes were shaped by grozing. The most interesting discovery was a large number of carefully cut and grozed geometrical shapes (triangles and parallelograms), which must have formed one or more windows in the nave of the main church at Tonovcov grad. They represent a predecessor of stained (painted) glass windows. A brief review of a selection of contemporary sites shows that glazed windows were not a rare occurrence in the Late Antique south-eastern Alpine settlements but that they appeared in probably all the churches and in most houses.