ATLAS – Updated Rules
Players may summon any tier of primarchs on their own castles. However, offensive actions using Silver 2, Gold 1, or Gold 2 primarchs are restricted unless the following conditions are fulfilled:
  1. The team must control at least 5 castles.
  2. At least 15% of castle guards must be deployed on 5 or more of those castles.
Exceptions:
Players are still permitted to attack with Silver 2, Gold 1, and Gold 2 primarchs without meeting the above conditions under the following circumstances:
  1. When the enemy primarch is occupying a castle owned by your own team.
  2. When the enemy is located in Red Zone territory (i.e., No Man’s Land).

Additional Information:

  1. You cannot defend an alliance owned castle if your team doesn't meet the 5 castles with 15% castle guard hired requirement
  2. All Primarch ability can be used regardless if you meet the requirements or not
  3. Offensive Actions for not meeting the requirement means that your ability to attack an enemy primarch and castle guard on a castle owned by an enemy team will be restricted and will show a "Can't Attack" message.

 

Reference:
Old Rules