The all too common concept of trying to "do Europe" is pretty unrealistic, and will most likely, if not ruin your vacation, then at least make it less enjoyable. While it is true that Europe is compact and easy to get around with efficient infrastructure set up everywhere, as previously mentioned, it also has more preserved history packed into it than virtually anywhere else. There are more than 400 World Heritage Sites on the continent, and that is just the very tip of the iceberg. So instead of running a mad dash through Europe in an attempt to get the ritual photos of you in front of the Colosseum, the Eiffel Tower, Big Ben etc. over and done with, the key is prioritize, pick 2–3 sights you really want to see per week, and plan a route from that. There are likely to be some amazing, world class sights and attractions that you haven't even thought about, somewhere in between two given cities, and finding those will – in all likelihood – be infinitely more rewarding than following the beaten down post card route.
Europe has been home to some of the world's most advanced civilisations, which has led to an astonishing cultural heritage today. Ancient Greece has been credited with the foundation of Western culture, and has been immensely influential on the language, politics, educational systems, philosophy, science, and arts of the European continent. Ancient Greek structures are scattered over Greece and Turkey, including Delphi, Olympia, Sparta, Ephesus, Lycia and of course the Parthenon in Athens.
Ancient Greece was followed by the Roman Empire, one of the greatest civilisations in the world, which took hold of large swathes of Europe, North Africa and the Middle East. Visiting Roman ruins in Rome is a no-brainer, with the magnificent Colosseum, Pantheon and the Roman Forum. Many Roman ruins can also be found in Spain, such as the remains at Merida, Italica, Segovia, Toledo and Tarragona. With 47 sites, Italy has the most UNESCO World Heritage Sites of any country in the world, directly followed by Spain with 43.