Walking south down Walbrook with the site of Bucklersbury House on the right. That side of the building worked alright; it was the side towards Queen Victoria Street that ignored the line of the pavement and created awkward spaces that was the problem, if I remember rightly.
I suppose the road follows the course of the old Walbrook river fairly closely, and no doubt it's in a pipe somewhere down below. There are people who get very excited about these things.
When they built Bucklersbury House in the 1950s they found a Temple of Mithras - used as I understand it a by kind of Roman Freemasonry with added animal sacrifices - along here; the remains were taken up and relaid (they were rather flat remains) in one of the abovementioned awkward spaces towards Queen Victoria Street.
The church of St Stephen (just visible in the second picture this side of the St*rb*cks sign) was originally on the west side of the Walbrook. It was rebuilt on the east side in the later middle ages, burnt down in the Great Fire in 1666 and rebuilt by Christopher Wren (unusually, for a City church, no one disputes the attribution). It's ingeniously domed and columned and generally revered by the kind of people who revere things. So worth a look inside, even if the modern rearrangement with the centrally placed altar is problematic, and the primitivist form the altar takes even more so.