Here is the requirement list for the Fortress. 

Raid daily and keep building troops.
Soldiers   - 50k +
Guardians  - 100k +
Rangers    - 200k +
Knights    - 400k +
Ambassador - 500k +
Fighters   - 600k +
Commanders - 1M +  & Up
Everyone needs to send 100 spies also!

Fortress Requirements

Fortresses are a great way of increasing the offence and defensive power to every member of the league.  Level 5 is the first safe stage and the knowledge levels can be upgraded to level 5 which is a good percentage for reducing food consumption and increasing troop power.  Tomes can ge obtained from league quests and league tournaments or purchasing them with sapphires.  Glory points can be obtained for league tournaments and purchasing them with sapphires. 

 

Fortresses do need to be defended with defensive troops.