Given the prohibition on entering a place of idol worship (Rambam, Peirush ha-Mishnayos, Avodah Zarah 1:3), when, if ever, is one permitted to visit a historical sightsite that has been used as a place of idol worship in the past if it's no longer used as such?
Potentially relevant factors might be the extent to which the sightsite is preserved (e.g. a fully standing church vs. the ruins of one) or the existence of religious symbols associated with idolatry (e.g. a pagan temple with statues of pagan gods vs. a place like Stonehenge which, while thought to have been used for religious purposes, bears no obvious indications of it)?