Ok, here is what I consider a complete list of instructions on how to get to
each of the 5 secret levels of Magic Carpet 2 (If I'm wrong, correct me!)
I) Level 5: Here, the hidden exit is quite easy to find. If it doesn't
show up immediatly after you defeat both enemies, just fly around,
kill some more creatures and collect some more mana until it does...
II) Level 8: After destroying the 4 structures that are your mission
objective, don't enter the regular exit, but leave the room via the
teleporter. A building appears directly in front of you. Destroy
and Enjoy...
III) Level 12: After picking up the Crater spell, use it to explore all the
rooms you can't normally reach... especially the one with the flashing
blue circle. Entering the circle reveals a teleporter that will take you
directly to the room with the hidden exit...
IV) Level 18: I wasn't able to pinpoint exactly what you have to do to reveal
the secret exit here... either you have to kill some more Wyverns than
just the one that is a mission objective, or you have to collect a large
amount of mana... but since to do one, you'll have to do the other,
that doesn't matter much, right?
V) Level 20: After killing the Hydra, fly to the lake; a building will
appear; destroy it! Nice and easy (except for that Hydra part,
maybe :-) )