Designed by Richard Lugar for the Crawshay family, the Castle was built in a year (1824-25) at a cost of £30,000, and has been a museum and school since 1910. Refurbished in 1992, to show its original Regency splendour, the building itself is well worth coming to see.

The entrance hall has a curiously Mock Tudor feel, planned to give the illusion of an ancient family home. The small shields around the frieze, the tall pillars around the windows, huge Gothic doors and the superb red and clear glass windows that fill the rear of the hall, all add to the imposing grandeur. The ceiling depicts the white rose of Yorkshire, the original home of the Crawshay family.

The atmospheric basement galleries are housed in what were once the wine cellars. They were refurbished in 1990 to create a gallery portraying 3000 years of the history of Merthyr Tydfil. Topics covered here include the martyrdom of Tydfil, the daughter of a local tribesman. The town takes its name from this Celtic saint, who was murdered by Pictish invaders whilst praying. The exhibitions move on to look at the farming history of the area, with a typical Welsh kitchen and dairy on display. The development of the iron industry is an important topic, with displays on the four major ironworks, Cyfarthfa, Dowlais, Penydarren and Plymouth. The living conditions for immigrants to the town are also explored with information about overcrowding and disease.

The story of Richard Trevithick's steam locomotive unfolds here too. It was the first locomotive ever to pull a load along rails. The engine was built at the Penydarren ironworks in 1804, 20 years before Stephenson's Rocket.

The political history of the County Borough is covered, with a look at the Merthyr Rising of 1831, and Dic Penderyn's prison cell. The development of the Labour movement is also portrayed on a series of information panels.