Introduction 

To emphasize one aspect from the start: In order to get a good understanding of Frankfurt's architecture one must differentiate between the starker direct city center and the drop-dead gorgeous city neighborhoods surrounding it. Combined, both illustrate a modern and traditional side, which blend together in a very fascinating/sometimes clashing sort of way (most visible in the Westend area).

The City Center (Main Train Station, Hauptwache, Zeil, Römer, Konstablerwache)

There's no denying it, the direct city center of Frankfurt is not what foreign visitors would normally expect of a traditional German city. It is predominantly modern and functional architecture (not necessarily ugly though) that can be found here. There is an obvious reason why Frankfurt's architecture does not reflect the history of the city: After World War II a big part (80%) of the city of Frankfurt had been destroyed. The city had to be rebuilt and its residents needed housing to live in. In the early 50s of the 20th century a lot of houses had to be bulit from 17 million tons of rubble. The Zeil is the perfect example of the purely functional ideal predominantly from the 1950's to 80's. By and by, Frankfurters are rebelling against the blasé attitude of project developers and architects. A reconstruction/retro movement can thus currently be witnessed (reconstruction of original Thurn & Taxis palace, rebuilding of Historisches Museum and a reconstruction of about 30 buildings in the old town ensemble). Some of the old buildings have already been rebuilt from plans and photos in the 1980's, so that the lovely Römer square in the city centre and the Alte Oper west of the centre can now be visited to get a glimpse into how Frankfurt looked before WW II. The Museumsufer on the south shore of the Main river is another worthwhile ensemble of old buildings (mostly containing museums for all kinds of special interests) that can be inspected. 

Word of Advice: Don't necessarily be thrown off by the 50's architecture as there are some true jewels housed in the inside. For example the fabulous Kleinmarkthalle (the farmers' market hall) can be found in a dull 50's style building, which once you walk in is like a completely different world and a must-see for all visitors. Another interesting building is the so-called IG Farben Haus, built in the 1920s in the northern Westend (Grünerburgweg). Once the headquarters of the infamous IG Farben (which produced and delivered poison to concentration camps during the Nazi era) and designed to make a visual impact it became the headquarters of the American army in Frankfurt after the war. In recent years, it has become a part of Johann Wolfgang Goethe Universität and can be visited by the interested public. In the mid 50s the first high-rise buildings were built (the first one being the so-called " Bienenkorbhaus" at Zeil in the city centre), but they were far from the skyscrapers that dominate today's appearance of the city.

The only Downtown Skyline in Europe (next to Rotterdam) - Hence the name "Mainhattan" (due to the Main River)

The upside of Frankfurt's love affair with modern buildings is that it has the ONLY inner-city skyline of Europe next to Rotterdam (London's Canary Wharf and Paris' Defense are too far outside the city center). This means that you will get spectacular views from visitors platforms (like the 257 m tall Main Tower or the Bar 22 in the Eurotheum). Being the financial capital of the third largest economy in the world, Frankfurt certainly is proud to show off the financial sector's wealth. Europe's tallest building, the Commerzbank-Tower is located in Frankfurt. and its most well-known skyscraper and landmark is still the 257m-tall Messeturm, designed by Helmut Jahn, one of Germany's most famous architects. As Frankfurt is Germany's financial center, most skyscrapers are office buildings of Germany's biggest banks and thus not accessible (except for the aforementioned MainTower).

Some argue that Frankfurt is losing its economic relevance in relation to boomtowns London and New York, BUT with the European Central Bank, the German Stock Exchange and four of the five biggest German banks having their seat shere, Frankfurt more likely will build more skyscrapers than less. Also, Frankfurt  has assembled all the big law firms, consultant groups and financial examiners in its city center, so the future looks rather bright.

Frankfurt's Lovely Old Residential Neighborhoods (Sachsenhausen, Bornheim, Holzhausenviertel, Westend)

Almost without exception Frankfurt's residential neighborhoods are true quality-of-life havens. Especially, Sachsenhausen and Bornheim are worth experiencing. Here you will see the 19th century era residetial homes with 3-4 meter high ceilings, wooden floors, lovely gardens and balconies and only a five minutes' bike ride to the skyscrapers. Definitely see the Schweizer Strasse/Textorstrasse in Sachsenhausen and the Berger Strasse/Helmholtzpark in Bornheim. In the summer these streets almost feel like parks, as the greenery tends to dominate. Also, the Oeder Weg is a nice little street for a more relaxed shopping atmosphere.

The areas around the parks Holzhausenpark, Grüneburgpark, Helmholtzpark and Güntherburgpark are simply amazing and very posh. Lots of small galeries, outdoor cafés and stores harmonize with the stately living quarters and villas. In the summer this has a very Italian feel.

You will definitely notice that every neighborhood has its very distinct orientation. Sachsenhausen and the Holzhausenviertel more of an old-money, traditional context (with all the open air apple wine gardens), Bornheim and Bockenheim more alternative, student/hipster oriented (with lots small shops) and the Westend oriented towards investment bankers (sushi, Italian dining, offices). Also, you will find many great restaurants in the residential neighborhoods. A local favorite for 30 years has been Café Grössenwahn (in the Nordend) or the Orfeos Erben (In Bockenheim). You can order anything on the menu and be blown away.

Very recommendable are the local theaters and culutral scene. The Holzhausenschloss has fantastic exhibitions and jazz. The Mousonturm in Bornheim has lots of great live music. The Stahlburg Theater is often ranked as one of the better satirical ensembles in Germany. The many small movie theaters Berger Kino, Harmonie Kino or Orfeo's Erben offer the typical independent movie niche almost always in English.

Conclusion

The first time visitor might be most impressed by the skyscrapers (and righfully so), but the true feel of Frankfurt has to be discovered in its fantastic residential quarters Sachsenhausen and Bornheim. Yes, the inner city is worth visiting with the Goethehaus, the Roemer and Alte Oper as highlights, but only visiting these places would do a disservice to the wonderful non-business side of Frankfurt. Having found an intriguing blend of modern and traditional architecture, Frankfurt has an ensemble that few cities have been able to achieve in this magnitude (maybe Boston). So here's the most important word of advice: visit and find out for yourselves!