Displaying posts from a Category where the Category ID or slug is not known

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WordPress uses the following hierarchy to determine which file should be used to display the category page:

  1. category-slug.php
  2. category-ID.php
  3. category.php
  4. archive.php
  5. index.php

Removing all of the “category-slug.php” and “category-ID.php” files whilst leaving just one file called “category.php” should do the trick.

http://codex.wordpress.org/Category_Templates