You need to put some cards for defense. Here is my suggestions:
- Improved Buildings - Buildings get +40% hit points,
- Flatland Castle - Castles (building which only hero can build) get +10% hit points and attack,
- Extensive Fortifications - Castles and Outpost get +25% hit points and build limit +5,
- Yamashiro - Castles get +6 LOS and range,
- Fort - Ships 1 Fort wagon,
- Vauban Fortress - Ships 1 Fort Wagon, adds 1 to Fort Build Limit,
- Castrametation - Ships 1 Fort Wagon, adds 1 to Fort Build Limit.