Barcelona offers a unique opportunity for the tourist on foot to walk from Roman remains to the medieval city, and then to the modern city with its open thoroughfares and grid-iron street pattern. The historic city center is fairly flat, while the modern city fans out towards the surrounding hills, bordered by steep streets that are vaguely reminiscent of those found in San Francisco. La Rambla near the waterfrontA notable feature is La Rambla, a boulevard that runs from the city centre to the waterfront, thronged with crowds until late at night and lined by florists, bird sellers in the higher part, craft sellers in the lowest, street entertainers, cafeterias and restaurants. Walking along La Rambla one can see the world-famous opera house El Liceu, the food market of La Boqueria and the Plaça Reial (literally Royal Square), with its arches and palm trees, amongst other interesting buildings. There's also a Wax Museum near the end. It is also worth keeping an eye out for pickpockets, for whom the boulevard is a favourite haunt.