Firstly, London has several major airports so flights to/from London are plentiful and have some of the lowest ticket prices.
Paris has always been a major tourist destination - and rightly so. In fact, Paris has been outpacing London for several years and many experts expect.
Istanbul has long been plagued by political instability and security concerns but things have calmed down considerably in the past few years.
Rome is one of the most visited cities in Europe as it is widely regarded as "the second most interesting city in the world historically, educationally and culturally".
Amsterdam is definitely one of the most expensive cities in Europe – especially when it comes to accommodation. Over-tourism is a problem in Amsterdam.
Barcelona finds its place on most travel itineraries. A vibrant food scene, unique historical architecture, great Mediterranean weather.
Vienna is a frequent top contender for most liveable city in Europe, so it's no surprise that it's a major tourist destination.
Milan is the fashion capital of Europe and the business/finance capital of Italy. It undoubtedly has a more buttoned-up vibe than the rest of Italy.
Athens is often the starting point to explore the famous Greek islands, but it is also the historical capital of Europe.