How do I beat level 9 of war for the overworld?

Solution 1:

Your enemies will focus on getting to the exit, and will run straight there through open corridors if a clear path exists. Fortunately, you know where the entrance is, and where the exit is, and when you explore, you'll learn that the path from entrance to exit has already been dug out.

Your job is simple: get in the way without completely blocking the path. When I played this level, I created a number of defenses, with a smattering of Beasts and other Minions to help hold the line. It almost doesn't matter what type of Minions you have (as long as there are plenty), but I recommend having a Barracks to train your troops up a bit – so you'll definitely have a few Gnarlings around to help hold the line. And add a Library so you can use some Sins to unlock more abilities.

The most important part of setting up traps for this incoming army is that you must not completely block the path. If you're going to add Ramparts (the walls that add to defenses' health) to a defense, don't add so many that you've completely blocked the path. I've had excellent results from having a front line of Blade Lotuses, with two rows of Bombards behind. Put these defenses in a section of tunnel that's at least three spaces across, so you'll have at least six Bombards in play. Make that tunnel wider, and you can fit in even more defenses (as long as you have the money and parts).

Schematics

If you have the time and funds, fill up a 5-wide hallway with traps, with a front line of Blade Lotuses and a three-deep backing of Bombards, preferably with a 3x3 Garrison room somewhere nearby to give your defenses a passive HP regen:

(This end towards enemy gateway.)
##LLLLL##
##BBBBB##
##BBBBB##
##BBBBB##
#### ####
#### #GGG
#### #GGG
####  GGG

(# is wall, L is Blade Lotus, B is Bombard, G is Garrison.)

The above might be your final shape. To get something in place quickly enough to help against the first wave, just go with a 3-wide section of hallway:

##LLL##
##BBB##
##BBB##
### ###

A Few Tips

If you have some extra funds and time left over, then yes, potions will help – especially since the entire defensive section will be in your own territory, so you can drop potions into the middle of a battle. The Frost Weaver potion becomes available in this level, and it creates an ice golem that slows down enemies and generally acts as a nice backstop for your traps. And the Flash Freeze potion is good for splitting your attackers into two smaller, more digestible groups: the enemies who are trapped (and will attack later), and the enemies who weren't hit by the potion and attack immediately.

Yes, you'll lose a few defenses along the way – you can heal damage with the Forge spell, prevent a little damage with a nearby Garrison, and then rebuild whatever gets destroyed anyway.

Finally, don't forget the Forge spell during construction (looks like a box): each casting of Forge is equivalent to one trap part, so two castings will get you a Blade Lotus or Bombard very quickly.

It also turns out that there's a Steam Guide for this level if you need a more-detailed walkthrough.