The rhomboid building on Plaza de la Seo is the entrance to the excavated remains of the substructures of Roman Caesaraugusta's forum, below ground level. What you mostly see are basements, foundations, sewers and drainage channels, with a limited collection of artefacts and explanatory panels in Spanish. An interesting multilingual 15-minute audiovisual show (on the hour) helps to breathe life into it all, and free audioguides are available.