On Rookgaard when you get a level you won:
10 oz
5 hitpoints
5 mana points
On mainland when you get a level you won:
Knight:
25 oz
15 hitpoints
5 mana points
Paladin
20 oz
10 hit points
15 mana points
Sorcerer/Druid
10 oz
5 hitpoints
30 mana points
So if you stay by every more level on Rookgaard you gonna lose hitpoint, mana points and oz (25 points in total).
Also all your skills advance more slow.