At a very early stage a contract was won to design and manufacture a Sound Level Indicator for the Open University. This contract eventually led to the supply of over 7000 meters and coupled with direct commercial sales of around 20,000 meters made the CS15C Sound Level Indicator the biggest selling sound level measurement product ever. From this beginning, a range of products were designed and to help Castle compete with big established competitors with multi-product catalogues, various agencies were acquired for several complimentary products.
By 1978 Castle had become well known and respected and had a growing order book. Unfortunately the market requirements had evolved faster than the designs emanating form the original engineering design partner, and in consequence they suffered a downturn in orders during the period leading up to the world-wide economic recession of 1979