You can obtain the Battle Fortress HQ skin only in the first 7 days of playing. The bonus is great and scales good throughout the whole game regardless if you have it equipped or not. It permanently gives:
- Troop Attack +3%
- Hero EXP Bonus +10%
- Zent Yields Bonus +10%
What is needed to get it
You must achieve 8 million combat power in the first 7 days of playing.
Is the Battle Fortress skin obtainable later?
No, unless you get it in the first 7 days this skin cannot be obtained later in the game even if you want to pay $ for it.
How to equip it
Click on your HQ building and click on Decoration.