Complex sentence: When Invader arrives in a city he obtains a map and spends at least a week installing the mosaics, which are catalogued (each given an identifier with the city code and sequential number), photographed and mapped to indicate their locations within the city.
Split into simpler sentences: When Invader arrives in a city he obtains a map and spends at least a week installing them. They are catalogued (each given an identifier with the city code and sequential number), photographed and mapped to indicate their locations within the city.