Only found at the death of a Ghost God, the Undead Lair (UDL) is a fairly easy Dungeon with lots of great loot for newer players and plenty of experience to gain. However, it is recommended that one have at least 400 hit points as instant death is common here due to traps.
An example layout can be seen below or visit this link: http://realmofthemadgod.wikia.com/wiki/File:Undeadlairmap.png
Most common to find will be Skeletons and Mummies. These are easy to kill. Circle them or Tunnel them up. If there are too many blocking a doorway, fire a few shots down the hall, then step to the side. Repeat this until the way is safe.
Next are Bats, Ghosts and Slimes, bats will dart to where you were last standing and are rediculously easy to deal with. The slimes however can be quite a problem, especially for those you have mid and short range weapons. Leave them to players with long range, if you'e not alone. If you are alone and have a class that can lob something at them, do so. If you want to risk it, the Brown Slimes can be circled. The Black Slimes can precede your movements, so as they're about to shoot again, move the other way while circling. Ghosts are somewhat alright to deal with so long as you take them on one at a time, if you are unprepared. If you're confident, you can run into the room, take down the closest ones, then back off. This may take practice, as with everything else, but it should work.
Sometimes you'll come across Constructs, Vampires or Reapers. The Constructs come in two varieties: Brown and Blue. Both can be dealt with by luring them near a doorway, and popping in and out of the doorway to shoot. If done correctly, you can kill them easily. Using abilities such as the Wizard, Assassin, Hunter or others that can deal high damage over walls will work fine in disposing them.
Finally there's Septavius. It is advised you don't fight him alone, unless you can dish out serious damage and can take a few hits. Avoid fighting him in the phase where he shoot many small white shots. These will pierce straight through your armor for a nasty 75 damage. There is then a phase with very large white shots. Avoid trying to combat him in this phase, as he can deal a ton of damage. Next is the phase with tiny brown shots and tan stars. The stars can confuse you, and the brown shots can quiet you. If you don't rely on Mana and don't care about the damage, go ahead and fight him in this phase. He will rotate between two shot forms during this phase. The first is when he fires three in a spread ahead of himself. The second form is when he fires two out from both sides. Make sure to clear out nearby rooms, as the enemies could come in and have a wonderful time forcing you to dodge more attacks.
Also keep an eye out on the floor pattern at all times. There are many traps that can do substantial damage littered about the place. You can identify what type a trap is by looking at it. You'll notice that the floor tiles with a circle will fire shots in all directions and more of a gap between the shots. The floor tiles with a square will fire a more focused shot with less space between the shots. These traps are very dangerous, and if you sit on one when it fires, you can only hope you'll survive. However, you can destroy them by dealing sufficient damage to it.