Nagas are Snake-Ladies and the only demons, other than Thalya, to have their own Research Slot (just above Hideout). And for good reason - they're the most useful demons in the game. They only have a little offensive capability, but their main purpose is: they're Healers. Simply keep them close to your attacking demons on the Overworld (but behind them so they don't get hit) and they'll automatically heal anyone getting hurt. They cost 100 Essence to unlock, and after that you should get 2 as soon as possible to add to your war party. With them on your team, as long as you've got a few fighters too you can attack pretty much any force and beat it - then just wait a few seconds and they'll heal your team (Thayla too) up completely. Essential. Note: They don't heal Undead, however.
Evil Essence areas are marked as little pools on your minimap, and there's usually around two close to your Dungeon entrance. The very, very first thing you need to do upon leaving your Dungeon is make for one of these Pools, slaughter the guarding Heroes, and claim it for your own (it'll happen as soon as they're dead). Why? Because it's your other resource, along with Gold, that allows you to unlock Research and Upgrades. Without it you're stuck with the absolute basics and you won't last long at just that level. And there's a more specific reason to do it fast...
I know it's tempting to do that old standby of making an eccentric death-corridor of traps to lure Heroes to their doom, especially as it was near-essential in the first Dungeons. In Dungeons 3 however, I very quickly stopped using them. They're tricky to produce, since there are a lot of prerequisites in their way in terms of Research, Boxes, Cost, Minions and more. You can't make a windy death corridor since there's already a direct path from the entrance to your Dungeonheart for Heroes. And once you learn that "F2" thing (above) you can simply throw your entire army at any invaders, which should stop them. All you really need is Doors to slow them down a bit, but you can stick a few Spike Traps behind them if you really feel like it.
This one's essential for defending your Dungeon, and we're shocked you're never told it - we only noticed it when browsing the controls. Which is this: pressing "F2" will immediately pick up every demon in your Dungeon, including Thalya but not including Little Snots. If you get attacked, simply press F2, aim behind or in front of the attackers, and press and hold Right Mouse Button to quickly drop your entire army on them - who should make short work of any invader. Note: this only picks up demons in the Dungeon, not on the Overworld.
Yes, you need the money, as per point number 4. But be cautious: if something's too good to be true, it's probably a trap. If you spot a big area covered in gold deposits ready to be mined, it will almost certainly be hiding something nasty. Dragons, Spiders, or even Dwarf Heroes might be hiding there. The good news is that most of the time they won't attack and will wait for you to attack first... most of the time. Sometimes however they'll immediately go around your Dungeon slaughtering the inhabitants. If you see a big patch of Gold, be ready to deal with some enemies.
In the same fashion you can turn on and off you video camera. You can either use the meeting setup panel (item 2 above) to disable your camera; or simply use the meeting control panel to enable video again.
For the longest time, you'll have ample amounts of Gold. Simply ask your Little Snots to dig out any Gold deposits you see, and massacre a few Good campsites, and you'll be overflowing with the stuff. Then, suddenly, you won't. You'll have dug out everything you can dig, you'll have destroyed all the feasible camps, and suddenly you'll start running low on Gold as Payday keeps coming around - and if your demons don't get paid, they go on strike. There are a few blocks of near-permanent Gold (or Diamond) reserves in the far corners of the Dungeon map, but otherwise you'll simply have to be aware. Further tips: Try not to let Thalya die, do not max out your demon army or Little Snots if possible, and you might not need to upgrade everything to max!
This also should go without saying but it needs stating. You may think you have things well in hand, but they can go wrong in an instant. I've had Thalya and a bunch of my forces cut down by a bunch of wandering Heroes coincidentally walking by just as I was recovering from an assault on the main Good Base, and watched in horror as they went straight to my Dungeonheart while I didn't have the Gold to resurrect Thalya or hire new minions. In short, Save Often, because defeat could be round the next corner if you're unlucky.
Seems obvious? Not if you haven't played these games before, and even I lost a few times because I forgot about it. The Dungeonheart is the big red crystal at the centre of your Dungeon, and the goal for every hero to attack. It's generally very powerful and takes a lot of hits to bring down, but if, say, you take your entire army to attack the Overworld and leave it undefended with no easy way of getting back, you might find your campaign comes to a grinding halt.
Apart from money, the one thing all your minions need is food. Gobbler Farms are the only source of food, and before you find ways to boost their efficiency you'll frequently run out. Consequently, you'll want to dig out a gigantic area for them and plop it down, so your guys will be well fed at all times. You'll still run out, trust me.