"Big ben" and "clock tower" both mean almost the same. According to the wiki page, Big Ben is the nickname for the Great Bell of the clock at the north end of the Palace of Westminster in London and is usually extended to refer to both the clock and the clock tower. It was the largest bell in the United Kingdom for 23 years. On the other hand, clock tower is a tower, typically forming part of a church or civic building, with a large clock at the top. I don't think "big ben" and "clock tower" tags should be aliased together, so I'll just leave them alone.