Architecture is amazing! I have never seen a mosque quite like it! And I live in the UK, we have lots of mosques. But even new mosques here are not extravagant like the one in Cologne.
I also did not know you can go inside for sightseeing like with churches of other faiths. I have always assumed it is closed for anyone outside the religion. I might be checking if that's on the table from now on!
I didn't know that there are also many mosques in the Uk, in Germany, too, especially in the West. We were also not sure whether we were allowed to enter, and therefore we asked again at the information desk, and she told us that we were allowed to enter with a head covering and that photos could be taken with cell phones, only cameras were not allowed:)