For a measure of the wealth that accumulated at the top of the industrial pile, check out this castle, built in 1825 by William Crawshay II, overlooking his ironworks. The house is now jam-packed with interesting stuff, from ancient Egyptian and Roman artefacts, to Laura Ashley and Julien Macdonald frocks. The basement houses an excellent exhibition on Merthyr's gritty history, taking in the struggles of the Chartists, trade unions and suffragettes. The house is surrounded by a beautiful public park.
Set into the hillside across the river from the castle are all that remains of the Cyfarthfa blast furnaces.