1- get ciele
2- put ciele as governor in the castle
3- upgraded ballista
4- Upgraded exploration to GM
5- Put Ciele as governor of eatch castle you destroy

You can get 4 castle on day 5 if you play hard mode because Xp level will increased faster for you too get exploration gm faster
You can get 4 castle on day 6 if you play medium mode

Surprise your Multiplayer Opponent with this kind of rush they will be amazed