Okay, my first entry is about the Cleric. Here are my ideas:
Level 0 :
The Cleric does NOT have his Fortress Power. He has NO sprinting ability. He does NOT have the red Hammer This may sound strange for a "level up", but this is the first level and his Fortress Power of healing himself will be a higher level up.
Level 1 :
He is now able to sprint
Level 2 :
He receives his Fortress Power
Level 3 :
He gains the knowledge about how to use the red hammer.
Level 4 :
His Speed increases a bit
Level 5 :
He gains immunity to all damage dealing Black Magic spells.
Level 6 :
His starting Armor improves
Level 7 :
His health slowly regenerates to 100 if less
Level 8 :
He gains fire immunity
Level 9 :
His Fortress Power adds 50 Health to each teammember aswell
...so this is my opinion about how the Cleric may improve.