It's the Eiffel Tower : like the Empire State Building or the Burj Khalifa. There are many towers in the world, but only one Eiffel Tower: hence the definite article. Bear in mind that, when it comes to English, the majority of 'examples on the net' are non-native. So, with an area as tricky as articles, many are likely to be wrong. If you happen to find an example without an article which is also correct, this will be because of the grammar of that particular sentence (as in my sentence above!).

The Eiffel Tower with the article