Level 7 is fish monster.  You get better draws and loot and can start getting Griffens(level 12) and Dragons (level 12) as well when you are city level 5.

There will be other perks on the in between levels that are not troop related directly (such as Items that can be used to boost or sabotage Troops waiting for battle in the arena).

Not every level is developed but I have troops up to level 150 coded in (http://ethereummining.co.in/game.php?action=troops#troops)