In 1637 Charles I turned the castle into a Royal mint, and it became a producer of silver coins. The mint closed down during the Civil War, but served as a warehouse for storing silver and lead. Oliver Cromwell slighted the castle in 1649. 

BNJ Reference: H. W. Morrieson,   ‘The coinage of Aberystwyth, 1637-42’, 10 (1913), 181-97, 3 pls.

Most of the images of threepences on this page have been supplied by David Greenhalgh.  Where DIG references are given these relate to the dies illustrated in his book, "The Galata Guide to Hammered and Milled Threepenny Pieces, 1551-1662".   Obverse dies are numbered 1-11, reverse dies: A-K.

Remains of the north tower gateway at Aberystwyth Castle.  CC BY-SA 4.0