Complex sentence: As contemporary Cardiff, location of the "Doctor Who" production, did not have enough Victorian architecture, the episode was filmed in Swansea, and computer generated imagery (CGI) was used as the main visual effect for the Gelth.
Split into simpler sentences: As contemporary Cardiff, location of the "Doctor Who" production, did not have enough Victorian architecture, the episode was filmed in Swansea. Computer generated imagery (CGI) was used as the main visual effect for the Gelth.