The Temple – Mithraeum – was the indoor meeting place for pursuing the Mysteries of Mithras. Remains of Mithraea (plural of Mithraeum) can be seen across many parts of the early first millennium CE Roman Empire – several websites are referenced in the Acknowledgments section.
Geographic locations such as Mackwiller, Alsace, France; Aquincum, Near Budapest, Hungary; Caernarfon, Gwynedd, Wales; Carrowburgh (Brocolitia), Northumberland, England; Duino, Trieste, Italy; Fertorakos, Hungary; Walbrook, London, England; Ostia Antica, near Rome, Italy; Rudchester (Vindobala), Northumberland, England and Savaria, Szombathely, Hungary can be visited but in addition other Mithraea are under current churches and many Mithraic altars and statues are in museums.
Details on their purpose, physical locations and construction are covered in the Temples of Mithras – Pursuing the Mysteries of Mithras PDF
As well as dating of Mithraea in London and by Hadrian’s Wall, much information complete with known or estimated construction dates has been found for several Mithraea beyond Britannia. Mithraeum Build Dates has two pages – individual Mithraeum dates and incidence of numbers of Mithraea by these build dates.