The steelworks of the Welsh ports were of obvious interest to the early Luftwaffe investigators. Iron and steel were strategic materials for all sides. The Dowlais steelworks in Cardiff was a huge target. In fact it was so big that it was difficult to work out exactly where to hit it to cause most damage. The largest high quality bombs were reserved for targets like this (500kg or more), with hard steel noses to penetrate concrete roofs. Incediary bombs were not very effective against industrial targets.
I cover Cardiff as a Luftwaffe target in Chapter 5 of Eye of the Eagle: Luftwaffe Intelligence and the South Wales Ports 1939-1941.

