Level 4 – The Cross
This level supports a few strategies. The High Ground in the center of the eponymous cross can be used for either your friend the Chilling Crypt, or a Burning Temple. Either way it will do significant damage. Temples go on the rocks with the mana pools, surprising absolutely nobody.
The second high ground should probably be a Crypt. You will also notice that while you can use a Den to ninja-proof the western entrance, the eastern entrance is instead surrounded by large amounts of useless snow. This is suboptimal, but you can compensate by using a lot of crypts, with their ranges staggered so as to not all shoot the first ninja they see at once.
A few towers should be added to the inner loop leading to your cave, for if – for when the ninjas make it through.
You will also be contending with mages on this level, and they may be worth a Frenzying, since your towers probably aren’t quite up to the task of breaking their barriers yet.
Level 5 – The Thicket
Another level with no real right or wrong answer.
Except not putting a tower on all four high grounds/mana pools, obviously. That would be a real wrong answer. But presumably you’ve taken care of that. There’s no one best mix of towers – the best strategy is to not mix your towers at all and just put Chilling Crypts on everyone. You should also set up two forward dens near the entrance, for double ninja-proofing. Don’t bother about a second line of defense, they should decloak well within range of the Chilling Crypt Doomsday Battery.
Level 6 – The Fortress
The obvious first move is to fill the northern ridge and its high ground and mana pool with Chilling Crypts.
The intelligent first move of the ninja-wary is to put a tower on the high ground immediately outside the entrance instead, and let it upgrade into something that could actually be a credible threat to an intruder who has breached your grand wall of Chilling Crypt doom. Any tower will work, but a Burning Temple works best in the close confines and open road. You should still grab the other mana pools, though.
After a couple of waves this will start to prove inadequate, so start bulking up your southern defences – the two ridges on either side of the high ground provide good places for a supplemental Temple and Den. When ninjas start to show up, set up a temple on the ridge near the entrance, to blast them as they enter.
Once your confident in the effectiveness of your southern defences – probably somewhere around wave 10 - go do the obvious thing again, and cackle as your enemies fall.