Organize categories

I have four categories: forum, news, video, events. Each of them has own sub-category: antiterror, design, security systems, private security.

I need: if a visitor chooses forum->antiterror in my menu, he comes to page (category.php) with “site.com/category/forum/antiterror” link and sees the posts which have “forum” and “antiterror” categories selected. By typing “site.com/category/antiterror” he can’t get anything.

But each gategory has to have unique name, so how to do that?