[totaltec]

This is just psuedo code, you should find and test for the actual variables to use:

{if $cat.categoryid eq 12}
  {*special behavior goes here*}
  {if $logged ne ''}
    {*Show category*}
  {else}
    {*Show message to log in with link*}
  {/if}
{else}
  {*Normal category behavior goes here*}
{/if}