Seems you can't swing a cat for enemy keepers challenging your domination of the underworld. Undeterred by the way you utterly smashed all resistance in Morkardar, not one but two keepers have set up shop here. In addition, heavy hero forces occupy the center of the realm - the very place you must build your dungeon, of course!
The first annoying thing about this realm is that you have no way to increase your forces, having no Prison or Graveyard. On the other hand, this means you get to have a lot of fun beating things to a mushy pulp. The second problem is that the large gold vein to your north is your only source of finance for the first leg of your effort here. There are gems far to your west, but if you go for them now, you will quickly discover just how strong the heroes of this realm are. Aside from the fact that you are entirely surrounded by a river of water, in which dwell too many level 3 Archers and Dwarves to count, there are caves of extremely strong heroes near the gems. We're talking level 10 Samurai and Knights here.
So, until you have researched Sight of Evil, use this old fashioned trick to avoid breaking into the river: dig out, claim, and fortify one tile at a time. Repeat doing this until you see the wall that your imp has created turn into a double-edged wall (look carefully at the top of the wall and you'll see what I mean). Any time a wall is built against empty space on the other side, it becomes double-edged. Using this knowledge to your advantage can make any realm where you need to dig about and fortify much easier.
Within your own domain you will find two level 10 Imps, trapped away within the giant gold seam, and a few blocks south of the left side of your portal is a hidden Increase Level special. Save this for later, however tempted you may be to turn your Imps into Level 3 speedsters.
When building your rooms, keep your training room 3x3. You will not have enough money to train more than a few of your creatures during the initial part of the level, and I recommend attempting to micromanage the training to make sure that the same few minions continue to get time. Otherwise, you'll end up with a lot of level 3 softies who aren't going to cut it. You really need your Dragons to be at least level 4, and preferably level 5 before you take on the next action.
As soon as you have the Guard Post and Bridge (your money cushion should still be fairly solid at this point), tunnel straight from your heart to the south until you come just short of breaking into the lake. After your imps have claimed up to the last tile, have them mine it out, quickly claim it, and then slap down a Guard Post on it. This will keep the Archers and Dwarves out, and it will allow you to build a bridge outward (sell the tile that connects to your dungeon, or your Guard Post will be for naught!).
You want to fortify all along the earthen walls to the south, because if you don't, the blue keeper will eventually break through here. Make one exception, however. When you reach the western-most block (eliminating any heroes who threaten your imps along the way), begin digging down along the impenetrable rock. Dig all the way to the south edge of the realm, stopping a few blocks short of the western-most edge. Quickly have your imps fortify this entire corridor so that the blue keeper can't get through. In the area behind your wall, a level 8 Horned Reaper is imprisoned. By walling this area off right now, you can save him here until later when you can afford to pay him. If you do not do this, the enemy keeper will get him, and that's simply unacceptable! Be sure not to release the Horny right now, though, or he will bankrupt you with his outrageous salary. If you accidently release him, reload from a recent save and try, try again.
Note that you must never sell your bridge along this southern wall. If you do, the blue keeper will be able to destroy the walls you have built simply by claiming tiles next to them!
After this ordeal, you can start to eliminate all the remaining heroes in the water. Snake your bridges around both sides of the realm, using any high-level Dragons and Demonspawn you have to kill the Archers and Dwarves a few at a time. Don't let your minions attract too much attention at once, and if they do, immediately pull them out of battle, let them heal, and then try to drop them in areas where they can fight isolated heroes. Once all of these dummies are dead, fortify the single earth tile you will find leading to the green keeper's area, just for safety.
The time is now upon you to claim your gems. Your money is running dangerously low by now, so much so that you may have had to sell your training room. That's fine because you'll be able to easily afford rebuilding it bigger and better soon.
But, you ask, how can you eliminate level 10 heroes with your level 5 Dragons? Well, you can't do that, of course. But what you can do is release the heroes in controlled groups and allow them to wander into the southern corridor of your dungeon, where you will of course have set up a number of doors with boulder traps behind them! By doing this four separate times, you can eliminate the archers, samurai, barbarians, and knights who dwell in the gem-area caverns relatively quickly and claim your riches.
Be sure that when the heroes are released, you have built a bridge up to your guard post, and also be sure that you have at least one extra door in reserve. When the heroes enter your dungeon, you need to slap a door down behind them. Otherwise, they may turn tail at the site of your boulder trap and run right out the door to safety, only to return moments later to your now undefended dungeon to wreak havoc! Sell and replace this extra door each time that you let some heroes in.
Hollow out the area near the gems and build an oddly shaped yet still highly efficient treasure room and start your imps mining. After you've built up a comfortable cushion of about 70000 gold, you can reconstruct a better training room, train all your minions as high as you wish, and release the fury of that captive Horned Reaper. In the caverns near the gems you will find a Steal Hero special. Saving and reloading is recommended until the special gives you a level 10 Samurai (you can increase the chances of this happening by using the special later when you have slaughtered most of the other heroes). Be sure to build a small separate lair for your Samurai so that they won't kill your low-level mistresses.
At any time you are free to begin an assault on the eastern hero keeps, of which there are two. The one to the south is the weakest, containing mostly level 8 Archers, Barbarians, and Giants. Within are two dungeon specials: another precious Increase Level and a questionably useful Make Safe. Immediately above this area is a small cave containing two level 7 Imps, and along the eastern rock wall is a wonderful Reveal Map special. You can get these even without attacking the hero keep, and so I recommend getting them as early as possible. Beware of the room inside the keep with the Increase Level special, as it has several traps that may hurt or kill your Imps.
When you reach the northern keep, the heroes within will be the only ones remaining. There is another Steal Hero special inside this area, but if you wait to claim it until after battling the heroes, it will be too late to use it effectively. So, instead, empty out and claim the outer corridor, then dig into the wall (though this wall appears solid, most of it will evaporate as you claim tiles next to it). Once you've reached the tile next to the area containing the special, fire off one shot of your Destroy Walls spell, which you have surely researched by now, and let your Imps get the Steal Hero special early. Use it now, and the one you got earlier if you did not already use it, and get those Samurai from within the keep. If you want, you can use lightning bolts to kill the rest of the heroes, or you can use your now powerful army to take them on face to face. Just be careful again, because this area is full of traps. Also, if you use lightning, avoid accidentally killing the unaligned Spider and Beetle in the room just to the east. If you get both of these guys, you can sacrifice them to create another high-level Mistress! Or you can keep them, but they are virtually worthless on their own...
Once all the heroes are eliminated, the time has come to turn your attentions against the impudent enemies that challenged you to this pointless display of raw power. You will notice that although the green keeper has a portal max of 30 creatures, he will never attain this. This is due to his conservative AI and a seeming inability to grasp the fact that his lair is too small. Any creatures he gets above 15 or 16 will get fed up and leave his dungeon, making him the weakest, and therefore the first enemy you should vanquish.
To make it pathetically easy, break into his largely undefended gem cavern with some concealed imps and claim as much as you can. Set down a Guard Post and let your powerful Mistresses scare away his imps. After they stop coming for a while, send your Imps to retrieve some lava traps from the Workshop and set as many as you can in front of his corridor to this area. If you can block off the entire thing, it will seriously hamper his cash supply. Unfortunately it will not starve him entirely, because this keeper knows how to sell items from his workshop to stay afloat.
If he tries to start a large battle while you are zapping away his imps, pull your mistresses out, sell your Guard Post, and wait a while before trying again. Do not let a large battle begin now, because despite outnumbering him, he will still do serious damage to your army that you cannot accept.
Once you've cut off his gems, let him wither for a while, and then try this devious little trick I have discovered that will work well in many realms. Take a look at his dungeon (you can see the whole thing because of the Reveal Map special). Look at the foolish placement of those boulders! Why, if you could set those off, you could crush most of his creatures in one fell blow... but, how can you do it? You can't get an Imp within shouting distance without it dying, or so you think, right?
Well, you can. Use two shots of the Destroy Walls spell on a tile next to one of his Boulder traps and then quickly spawn an Imp on the new dirt tile that has appeared. The Imp will get frightened and run to his doom, unleashing the Boulder trap in the process! Enjoy as it rumbles about the enemy's lair, crushing his precious monsters. You can keep doing this over and over as long as his Imps haven't claimed your dirt tile. And if they do, you can always try making another one on an adjacent tile. Repeat this as much as is practical and you will have his army cut down to nearly nothing.
Start claiming away his dungeon and advance your Guard Posts to keep his imps in check. Use fresh level 1 Imps to do all claiming in this area, since this keeper has many dangerous traps, including Boulders, scattered everywhere. Once you have brought this pathetic fool to his knees, finish him off in style with a massive final battle. One down, one to go, and barely a sweat broken.
Whenever you are ready, you may now turn your attentions against the blue keeper to the south. He is significantly more powerful, and his AI also has an annoying bug that all of its type share. It is allowed to illegally place traps, never recognizes that traps have been filled, and overbuilds its workshop to ridiculous proportions. So, you will find tiles in his dungeon which contain 50 lightning traps, 10 boulders, or numerous lava traps (which cause a brilliant looking but harmless light show if you should happen to trigger them -- build and sell a bridge over the tile to put a stop to it).
It's easiest to use Lightning and Cave-In to kill off some of his Imps and then try to make a foray into his dungeon from the southern tunnel you made earlier. You can use the same trick of triggering Boulder traps inside his dungeon, but it will not do nearly as much damage to him as it did to the green keeper, and you also want to avoid giving him too many dead bodies. He is allowed to make vampires, and if you start leaving dead carcasses all over his dungeon, he will in very short time have raised a legion of them. And he can train them shockingly fast, as well.
The best protocol is to trigger off his boulders using expendable Imps and then immediately invade with your whole army. Use your spells liberally since this is the final hurrah. Beat it to the heart room and then beat the heart into oblivion. This was a protracted battle, but still not really that difficult. Things will change soon, though, and much for the worse.