I would argue that Lisbon is hillier than even San Francisco! Whew! The pitches of many streets is quite intense, hence the tram system up select streets in Lisbon.
Lisbon’s hills and trams
Lisbon Trams and hills
On many very hilly cobblestone sections, the stones are worn down to a glossy sheen which is extremely slippery.
Slippery streets in Lisbon (photo from Sintra but best example so you can see….)
Even with rubber soled shoes, it was hard to get a grip on the surface at times and I slipped more than once on the street while descending. Be forewarned!