There are four final crowns at the tops of four towers:

Tower 1: North of Horne, and the Witches manor.

Tower 2: Look on the map, in the water area, south of Invidia and north of Tower 1, there is Tower 2, you'll have to take your ship to get to it.

Tower 3: Located in the town of Liberte, on your boat at the northern most part of the town

Tower 4: Located on the landmass in the north-east around Spelvia.

All of the towers are locked by gold locks that takes a key that you recieve from Rolan late in the game.

The four secret crowns are:

Ninja

Special Ability - Encounter fewer enemies

The abilities you'll learn as you level up the crown allow you to better evade your enemies.

Sage

Special Ability - Enhance the effects of white and black Magic

The abilities you'll learn as you level up the crown will allow you to lower the cost of Black AND white magic by 1

Scribe

Special Ability - Grow more powerful the more time you spend playing

The abilities you'll learn as you level up the crown will be based on time damage dealt by the time you've played, healing allies health based on crown levels, etc...

Storyteller

???