Warcraft III: The Frozen Throne Walkthrough & Game Guide

Mission 6: A New Power in Lordaeron
The next day on the outskirts of loraderon's capital city, Slyvanas and her allies
prepare to lay siege to Balnazzar's formidable defenses.

This one shouldn't take you that long.   You really only need to know what to
build and how to execute this.   With the undead base, build crypt fiends and
destroyers.  If you haven't made a destroyer yet, you need to get obsidian statues
and a black citadel and research the destroyer upgrade.  These are powerful anti-
casters.  If destroyers aren't your forte, you can get frost wyrms, although they
are much more expensive and don't serve the same purpose.   Don't forget to
upgrade the knights and the creature tech tree for undead.

With the human base you should get knights, knights and more knights.  However,
you need to do the optional quest to make this mission very easy.    You'll get
siege engines and mortars which make this mission pretty simple, as they destroy
all air units with ease (making the work of the crypt fiends basically overkill),
and can take out buildings very quickly.

I would spend the first part of the mission etching up.    When you have enough
resources upgrade to black citadel and castle. Make sure you build a few ziggurats
at the undead base.   Eventually you'll learn about the optional quest.

Optional Quest: Ironforge Dwarves
-Locate the missing dwarves
Really  not all that difficult, and very much worth it.   You can either go right
away with your forces from Garithos' army or you can wait and get air (I got
destroyers) and have them fly over to the east.  They can  kill all of the kobolds
without even getting hit, with the exception of the geomancer.  Then they can head
south and take out the other group of kobolds in addition to breaking open the
cages to get even more riflemen.  The only thing  that garithos has to do is kill
the golems to the north of those caged riflemen and break open the rock wall to
the east of the dwarves (the destroyers can do this too).   Simple eh?  You get a
lot of food from those dwarves, so you can have a distinct advantage over the
undead if you have a lot of food before you take the dwarves (you can exceed the
food limit this way).  I had around 80 food with the undead, and around 100 with
the human and the purple base was pretty easy.  So when you have enough units head
into the purple base (it's in then middle).   Make sure you pay attention and
charm powerful units like frost wyrms before the siege engines kill them.  Use
doom early on so you can have an early advantage.

If they attack your base you should probably just ignore it if you are winning as
you can mop up the base fairly quickly.  When the purple base is destroyed
entirely, the mission will be complete.