At a strategic location on the banks of the river Pisuerga stands the castle of Simancas, from the 15th century, considered to be one of the most important in Spain and which is an emblematic feature of the town.
The fortress, which was built by the Admirals of Castile, was later handed over to the Crown and has housed the General Archives of the Kingdom since the time of Felipe II. It contains an interesting collection of documentary gems on monarchic rights and imperial administration. It currently houses the General Archive of Simancas, which gathers together important documents of state, as well as being used as a centre of investigation. Also in town you can admire the reredos and the valuable silverwork of the monstrance in the parish church.