This is a silver real
(royal) from the Kingdom of Castile
and Leon in Spain. This coin was used during the reign of Pedro of Castile
(note the "P" on the
front and the coat of arms
on the back),
who lived and died within Chaucer's lifetime.
Some other types of currency used in Chaucer's time were gold florins
and the gold
introduced by Edward III.
Built in 1364 by Pedro of Castile, Alcazar (in present day Seville, Spain)
is the oldest royal residence in Europe that is still in use. It was beautifully
built in the Moorish style.