Dangerously addictive, but worth playing.
Managed to get all brilliants on level 21. I think it should be possible to get it even earlier, but Ninja boss on 14...
Anyway, to add to everyone else's tips:
As in any tower defense, clever combinations will quickly win levels. Crypts are wonderful supports due to freeze and fear (blaze and poison cloud are fairly bleh), but if they don't wipe out the enemy in one hit, they look fairly silly as the enemies wander past. Ballistas are of poor accuracy, but freeze an enemy and they're sitting ducks. Burning temples look cool and work wonders, but are really slow - try setting up chilling crypts to scare enemies back up into them. The demon towers have the most power, but need to be supported due to their poor range. The orc towers are probably the least flexible, but the splash damage, decent range and good rate of the ballista towers are worth waiting around for.
The basic principle is - establish only a few towers (maximum of 8 per level) of different types, start from the gems and work your way outwards, and start upgrading from the gems outwards again, keeping an eye on your mana. Too little mana will render you vulnerable to insufficient meteors against ninjas or frenzies against assassins, too much mana will be wasted and should be cut awayed into gold. Don't think you need to build before each wave as enemies randomize where in the maze they come in from. Don't bother with the "final form" of any of the towers - with the exception of fear crypts, the final forms turn up too late in the levels to be that useful. Upgrade chilling crypts first as these will maximise the effect of all the other towers, as enemies will be kept busy running backwards - this will incidentally increase the experience other towers receive.
Upgrades: Try to get freeze and max fear as quickly as possible. Cut away upgrades will give you a way to convert mana to gold, which is crucial for upgrading your towers later in levels.
And to repeat all the ninja boss techniques: Max meteor, max mana. Banshee crypts won't work as the smoke bomb activates on the first hit. You need to hit them hard as soon as they turn up with hellfires, fear crypts and ballistas for lots of splash damage and immediate damage, and then meteor the rest out of them. If you are very lucky they will fear as soon as they turn up. Unfortunately there really is a luck factor and you may have to replay a level a few times to get through. To guarantee death, upgrade "adoration", and you can build a few temples after the dread level 14 boss turns up to get enough mana for 6 meteor strikes.
Okay, that's it. I'm off to reclaim what's left of my life. Do not make another, life is too short for me to play another one of these...
