The Eastgate Viewing Chamber is the remains of the Roman settlement. While it can be viewed at street level from outside of Boots, there are also tours from Museum of Gloucester that can be booked.
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Worth a visit for the Roman artefacts, Roman kitchen and medieval street.
Don't pass up the opportunity to visit the tailor's house from Beatrix Potter's 1903 story "The Tailor of Gloucester". Situated a stone's throw from Gloucester Cathedral down a narrow alleyway.