OOoh, Last comment with my last few ideas:
Perhaps add a way to get on top of the castle walls, And a way to aim your spell somewhere other than directly in front of you?
And instead of right clicking to regen mana, You could add a full-time mana regen
That regens your mana when not in combat
That way you could use right clicking and dragging to control the camera.
AND
When you get to the point of adding different elements for mages
Make it so fire wizards set you on fire with their attacks
And if a friendly or enemy water wizard uses their water thrower spell on you, It puts you out :D
Also, Some ideas for cleric/ranger
Ranger:
Could use a longbow in both hands or a crossbow with a kiteshield for more defense.
Could maybe have different types of arrows like fire arrows or ice arrows
Cleric:
These guys call upon the powers of the gods so they will be very good support for their teammates
Could switch their powers to become curses upon the enemy, Like making them weaker.
How about collectable weapons? Inventory system, Maybe a shop where you purchase things with coins earned from killing other players? Name-tags over people heads? Maybe a level up system that goes my how many kills you have? And maybe invert the y axis camera control so when your mouse goes up so does the camera. I like the idea of the four basic classes, But I want them to be more customizable. Such as you could be a fighter class but not constrained to just using a sword and shield, You could dual wield or fist fight. Mages could have choosable elements, Like fire, ice, lightning, earth, air.
Maybe more maps/ a map build, A game lobby so you can choose what game you enter...
 "Ricochet Kills"){kind=icon}
 "Medieval Arena"){kind=icon}
 "Mutilate-a-doll"){kind=icon}