NOTE: none of these tricks will work if you enter a cheat code or alter
the settings once the level has started, and most won't work unless fog
of war is on and the mouse is set for WAR I style.
        Amazon: found in the forest, expansion level 1: you have to send
Alleria out due north immediately following the start of the level, and
using ONLY ALLERIA defeat all the forces at that first intersection.  If
any of your other troops gets a kill or is wounded, it will not work.  If
you are successful, and do so in time, the Amazon Spirit will appear
(looks just like the paladin's Holy Vision sparkle, but it moves around.)
 When it hits Alleria, she upgrades from ranger to Amazon.  There's no
noticeable change in stats, but she fires her crossbow shafts roughly 3x
as fast as Alleria did, plus she can call Neko later on.
        Neko: found in the forest, expansion level 1: once you have the
amazon send her solo to Danath's farm in the southwest corner of the map.
 Danath won't appear right away, so you have to attack the farm until he
comes out, just before it bursts into flames.  He'll start attacking, and
if you beat him he'll turn into Neko and the real Danath will reappear. 
Neko will go rescue passive, so he too will join your troops.  In later
levels he must be summoned by having the amazon Alleria click the "Neko"
button, located right below the "Attack" button.
        Spirit of Lothar: summoned at circle of power, any level after he
dies: This trick requires a fully-pumped army.  After completely
upgrading all spells, shields, arrows, swords, and ballista shafts, place
a wounded footman on the circle of power and surround him with three
mages, one to the north, one to the south-east, and one to the
south-west.  Note that this footman must have less than 5 hp left, or the
trick will botch and the mages will be killed.  First have the northern
mage cast "invisibility" on the other two mages, then flame shield on the
footman.  Next, quickly have the south-east mage cast invisibility on the
northern mage and blizzard on the footman.  Finally, before the
invisibility wears off, have the southwest mage cast PolyMorph on the
footman, who will have been skeletonized (not killed!) by the blizzard. 
You have to act freakishly quick on this last bit, or the skeleton too
will die and you'll have to start over.  If all goes well the mages will
explode and the skeleton will transform into the spirit of Lothar.  Make
sure you have a paladin nearby, however: he'll only have 1 hp to start
with, and if he dies before the level ends, you'll never get him back. 
However, if he does survive, at the beginning of every following level he
will replace one of your starting characters until he is destroyed.
        Buffy the Vampire Slayer: found in the twisting nether, expansion
level 3.5: now this one takes some doing.  Using Turalyon, you have to
get him to the Dark Portal with everybody dead.  If anyone else is alive
the attempt will fail and the level will end as usual. There is an order
to be followed, although if you can find another way it will probably
work just as well: kill all your enemies in your usual way, then kill off
everyone and everything except for Turalyon, a transport, and either an
elf or a ballista.  Load the two land-dwellers onto the transport, and
unload them by the dark portal.  have the elf/ballista destroy the
transport, then have Turalyon destroy the elf/ballista.  With Turalyon
alone remaining, he can enter the dark portal by simply ordering him to
move on it.  This will allow him into a new level, 3.5, where there is a
dark portal on either side of a road, and Dæmons all about.  If he goes
through the opposite portal he'll warp to a later stage, but if he
charges through the Dæmons to the east, he'll eventually come to a circle
of power containing Gul'dan, Lothar (even if you freed his spirit, you
still had to just kill him), some shadow-guy, and Buffy.  The other three
will attack you, but Buffy can be rescued and can kill the Dæmons. 
Turalyon can go out the first portal, but Buffy has to go out the second
portal beforehand, or else she won't come back into normal play.  Once
both have left the stage it will end, and Buffy will awake in a graveyard
(see her pud: I replaced Buffy with Alleria due to a little problem with
the translation, but it is otherwise accurate) and if she can defeat all
the enemies and secure the inner sanctum she'll appear in level 4 and on
as another hero.
        Ner'zhul: found near the new portal, expansion level last:  This
one requires ridiculous timing.  He's only there for about 30 seconds,
and then he enters the portal, so you have to tag him before he goes
through.  If you are successful it won't matter if your troops fall,
he'll still start off in a bizarre level manned entirely by cheap
sappers.  Again I replaced the heroes for Pyro.pud, but to really get the
feel for this'n you hafta play it out the right way. 
        Tick and Speak: further evidence of a weird mind.  You remember
that Pud on the expansion CD named suicide?  the one that says "you
survive, you are W2 God(dess)!"  Well, if you survive, this is the next
level.  Uncommon for a pud to have a level 2, but this one does, some
sorta secret bonus.  It's much easier to beat with Tick and Speak, but
the method of summoning Speak to attack just like Eye of Kilrogg and then
letting 'er rip takes a while to master.  My tip for beating suicide.pud:
move around.  It's tough, but you can get the catapults to blow up the
grunts if you're quick enough, and after a while they just run away!