And how exciting would it have been to witness Filippo Brunelleschi demonstrating linear perspective for the first time in his baptistry in Florence.
A scenic park, the Valkhof ("Falcon's Court"), contains ruins of Charlemagne's castle, which was destroyed by the Vikings but rebuilt by Frederick Barbarossa in 1155 before being demolished by French Revolutionary troops in 1796; a 16-sided baptistry (consecrated in 799) and the choir of its 12th-century church remain.
The key, a handsome, square-topped Medeco with an arty logo, opens locks at two dozen locations in the five boroughs, from the baptistry at the Cathedral Church of St. John the Divine in Manhattan to a locker at Gleason's Gym in Brooklyn to a very private and humble room (no spoilers) at the Louis Armstrong House Museum in Queens.
"I've always had a passion for history," Mr. DeRuiter said the other day, sitting in the cathedral's baptistry beneath the colorful Peter Stuyvesant heraldry, which honors the colony's last Dutch director general.
Then it's on up to the Duomo and baptistry (Battistero di San Giovanni), on a route that's higgledy-piggledy, very up and down, but a beautiful way to experience the city.
Most famous Campo dei Miracoli Medieval Italy's most ambitious building programme: cathedral, baptistry and bell tower arranged with precision on a broad expanse of green lawn, these days hemmed in by endless and exuberantly kitsch souvenir stalls.
The route takes me past an ancient Roman bathhouse and Christian baptistry, before leading me up steps worn smooth by centuries of shuffling feet to the magnificent Venetian castle.
Diocletian's mausoleum became the cathedral of St Domnius, the Temple of Venus was converted into a baptistry.
At its heart is the 12th-century duomo and the pink-marbled octagonal baptistry, frescoed marvels that tell every biblical tale from old testament to new and back again.
A tour of the historic centre – starting with the duomo, baptistry, and painted ceiling at San Paolo convent, then heading over the river via Palazzo della Pilotta to the Palazzo Ducale – is fascinating and not too demanding.
Tour buses have always made a beeline for Pisa's Campo dei Miracoli ("field of dreams") - the great, grassy compound where the tower, cathedral and baptistry form one of the most photographed set pieces in Italy.
