World War II, the Berlin Wall. Remember the Wall only came down in 1990ish. There is still a huge swath cut thru the city where it used to be, without buildings. The former east german side has many old buildings built up in the prosperous pre-war era, that are now rather inexpensive.
In the past 20 years things have improved quite a bit in terms of renovation, but it takes time for population to build up again, and thus the costs are still low because there is abundant supply.
But a lack of buildings (due to the Wall) should mean rents are higher. Same for bombing destroying buildings, this results in fewer building on the market.
As to pre-war architecture, all of Europe, including Eastern Europe is full of it.
The only thing which might give me some insight is that you mention it takes time for the population to build back up?
So rents are cheap because Berlin is today still under populated?
But why is that? Or why are other formerly bombed out cities not also under populated? In Easter Europe there is still a lot of migration from the smaller cities and villages into the big cities. This may be why those are more expensive than Berlin. But that only begs the question why is Berlin specifically so slow to re-populate? Is it because other German large cities, even with higher rent, are more attractive? Which would explain why the rent there is higher?
In the past 20 years things have improved quite a bit in terms of renovation, but it takes time for population to build up again, and thus the costs are still low because there is abundant supply.