Complex sentence: The Roman Catholic Church erected cathedrals in Roskilde and in Lund (in what is now Sweden), which laid the basis for further development in those regional centres, and as Copenhagen was midway between the two cities, it was centrally located for traffic and trading.
Split into simpler sentences: The Roman Catholic Church erected cathedrals in Roskilde and in Lund (in what is now Sweden), which laid the basis for further development in those regional centres. As Copenhagen was midway between the two cities, it was centrally located for traffic and trading.