The literate Temples quote from Richard Lovelace's 1642 poem "To Althea, from Prison" ("Stone walls do not a prison make,/Nor iron bars a cage"). He is, however, believed to have written it whilst at Westminster's Gatehouse Prison, not Canterbury.
Rather than the boat tour-guide's estimate of "about 1208", it is believed that Greyfriars (the first monastery built in England by Franciscan monks) was established in 1267.