What does expanding your castle do?
You are told "good things may happen" if you expand your castle, but beyond the associated Plaza Tickets, what effect does your castle size have?
There are 20 levels to your castle. The first few unlock abilities/perks. Subsequent castle upgrades update these abilities. The abilities you can unlock & upgrade are:
- Ability to hire batches of soldiers using Play Coins
- Receive Advice (i.e. Have Wentworth split up your forces automatically)
- Spy on the Enemy to find out which type of unit you will face next
- Conversion Rate - Upgrades how many soldiers join your forces from the enemy side upon victory.
Unlocks:
- Hire soldiers - Small batch for 5 coins
- Receive Advice - from Wentworth on your army composition
- Hire soldiers - Medium Batch for 10 coins
- Hire soldiers - Large batch for 15 coins
- Spy on the enemy - See which troops will be deployed first
Upgrades
- Hire Upgrade - Doubles the number of soldiers in all batches
- Hire Upgrade - Triples the number of soldiers in all batches
- Advice Upgrade - Receive advice up to two times
- Hire Upgrade - Initial Size x 4
- Hire Upgrade - Initial Size x 6
- Spy Upgrade - Spy up to two times
- Conversion Upgrade - Increases likelihood of getting soldiers from foreign kings
- Hire Upgrade - Initial Size x 8
- Advice Upgrade - Receive advice up to three times
- Hire Upgrade - Initial Size x 11
- Spy Upgrade - Spy up to three times
- Conversion Upgrade - Further increases likelihood of getting soldiers from foreign kings
- Hire Upgrade - Initial Size x 12
- Advice Upgrade - Receive advice as often as you'd like
- Spy Upgrade - Spy as often as you'd like
Source: GameFAQs
Expanding once gives you a ticket for a hat. Expending a second time allows you to hire 300 troops for ten coins. That's all I've done so far.