There is no strict city border (like Moscow Ring Road, though it’s not true for Moscow anymore due to recent border enchancement). Streets are clean and I was surprised that I met a lot of tourists. Even some Russians.
Though most of the buildings are modern. Just a few blocks are from the old times (I wasn’t there yet). One of old building is Alter Oper (was built more than a century ago, but was damaged during WWII and restored only in 1980s).

There are many hight scrapers in the cite centre:

One of the popular tourist sight is Main Tower.

5 euro and you are on top of this building. 200+ meters high =)

Then there is train station and Deutsche Bank twin towers. It looks much more interesting from top

Despite popular opinion that people in Europe go sleep at 8 in the evening, it's quite crowdy till about 11. A lot of cyclists, runners. Europe is being healthy :)
A couple of photos of central walking street Zeil at about 11 in the evening. It's something like Moscow Arbat, but a bit more crowdy. Though it also depends on the day; The day before yesterday it was so full, I barely could pass it. Yesterday it was just like on a photo:

There are too many wild animals inside the city. In the park right next to Deutsche Bank twin towers I met rabbits family (not just one lost rabbit, but the whole family, who lives there):

The view from the river is very picturesque in the night time: