The City of London, the literal and figurative “square mile” that was once London’s medieval core and now serves, with Wall Street, as one of the global capitals of finance, is a dense potpourri of architectural styles. Tudor wood and stone (or, at least, what little of it survived the Great Fire of 1666 and then the onslaught of the Luftwaffe) huddles in the shade of sharp and thrusting twenty-first-century glass and steel. When it comes to the City’s more historic structures, it is difficult to swing a dead cat without knocking it against something Wren had a hand in…