Simplify the following text for a Grade 7 native English-speaking person.
Original: From 1966 to 1975 the Gropiusstadt was built , a `` Trabantenstadt '' or city-within-a-city housing estate , designed by architect Walter Gropius .
Simplified: Between 1966 and 1975 , the `` Gropiusstadt '' was built . This city-within-a-city was designed by architect Walter Gropius .