Below the fortified bridge are the remains of the Jewish synagogue and
mikvah (ritual bath). The mikvah is behind a locked gate and can be seen only at certain times with a guide - ask at tourist information for times). The ruins are clearly marked and helpful signs show you where each room would have been. From the ruins there are good views of the fortified bridge and river Fluvía.
There is an easy walk which leads from the ruins and goes alongside the river to just past the fortified bridge.
The synagogue/mikvah is well signposted from the praça de libretat.
Google map: tinyurl.com/ye66gdq