How to unlock the corner maps in Gemcraft Labyrinth

I was one of the first people to beat endurance mode (level 1337) on Armor Games' leaderboards. It's not that hard once you have it set up right.

I'll re-paste the mini-guide I posted on JIG here:

The first key is to get a gem that is Yellow-Orange-Lime... in a trap. You'll have to 8x-Amp a single tower for a while before you have a high enough gem, but once your YOL gem is about grade 12 or 13 you can put it in front of the entrance and instantly fill your mana bar with dead monsters.

The second key is to use Giants Only and summon as many as you can handle each wave while you're still setting up. For me, it was 1 gem grade per 10 waves, so 1-10 is a grade 1 gem, 11-20 is grade 2, etc. You make a huge profit by doing this, since the giants are worth so much per kill. Do not summon after 160 or so, since the natural HP of the creeps quickly starts to climb to 3.33 billion. The nice thing about this is that the max natural creep HP stays at 3.33 billion for the rest of the game.

Other important tips:

• Put a blue/purple trap before your YOL trap until you're comfortable that your YOL can kill anything (grade 20-21 or so).
• Remember: Don't summon after round 160 or so. You've been warned.
• Get your amulets done early on in the battle. For example, it takes about 4-6 grade 19s in a shrine to kill off a monster with 3 billion HP, so save yourself the trouble.

I did this for the first time on H11, and ended up with a total multiplier of 7583.

Firstly you get some extra monsters to kill. Also as JonathanJ mentioned you get 10% in your summon multiplier for every tomb you open. Note that all tombs are not the same. There are tombs with higher hit points and armor and the monsters they spawn are tougher to beat (Higher hit points and armor).

Generally it is good to avoid opening the tombs the first time you play a map. You will have the opportunity to come back and replay the map with higher multiplier settings when you reach a higher level.