The roof of St.Peter's in the Vatican is accessible by a lift plenty big enough to take a wheelchair and someone pushing it.  The roof is brick-paved and has some steepish slopes, but the averagely fit carer should be able to get a wheelchair user around up there.  And it's not just a roof with wonderful views - there are loos, a drinking fountain, a shop, a post box and a cafe!  It seems that comparatively few people get up there, too.