Simplify the following text for a Grade 7 native English-speaking person.
Original: It was then located on the territory of Lower Lorraine , in the area of Geldern and Roermond , with its main stronghold at Montfort ( built 1260 ) .
Simplified: It was then located on the territory of Lower Lorraine , in the area of Geldern and Roermond . Its main stronghold was at Montfort ( built 1260 ) .