Not the most monumental city in the region, but...
Castelló is not a monumental town, at least not so much in terms of other historic Spanish cities and towns. However, it is quiet old (in 2002 it celebrated its 750th anniversary) and has preserved a few interesting buildings of its historical legacy, as well as some picturesque districts with low-rise, bright coloured buildings, where a stroll is most pleasurable.
Urban speculation, though, has brought havoc to parts of the city, which seem to have been carelessly designed.
Very few tourists will use Castelló as a base for their explorations in the abrupt Maestrat region or the sun-drenched Orange-Blossom Coast, as there are many other more interesting places in the area for that purpose. Besides, Valencia is too close as to consider it the real urban hub of the region. In spite of this all, Castelló is interesting enough to spend a day or two, should time allow.