Clearing the secret level

You'll get 500 gems as the clear reward!

Runes

First off, you'll want to clear with at least 4 runes to unlock endless mode. These are the ones that I chose the first time I succeeded. The second time(Which is when I'm making this guide) I added the 1 life only rune.

Upgrades and loadout

We want to have at least level 10 Harpoon Trap since we don't need the tier 4
level 1 Push Trap is good enough
Mandatory ones to have at level 15 are Blackhole Trap, Slime Trap, Barricade, and Spike Trap.
Recommend at least level 40 Spring Trap
Inferno Trap is entirely optional
Last one is up to you.
I had lvl 15 in starting bonus gold.

Choose whichever consumable you like the best. I took snare.

Wave 1

Pre-wave Setup
Use the barricade to squash a stray or two, and the use Snare just in case another leaks. Spend the money to fill in the top and left wall with Harpoons.

Wave 2

By wave 2 you should already be filthy rich. Add another Spring Trap and upgrade them all to level 3, with the rightmost one at level 4. I forgot and only did this a lot later. Barricade the left of your portal, and spam Push Traps on the right.
Before wave 2 ends set up a level 4 Slime Trap, one Inferno Trap and 3 Spike Traps. Upgrade the Inferno and Spike Traps to level 3 asap.

Wave 3

Spam Blackhole Traps and Push Traps on the right. This lessens the workload of the Spring and Harpoon Traps by about half
Upgrade the first Barricade to be invulnerable before the end of this wave

Wave 4

Spam some Harpoons and upgrade the older ones. Be sure to keep around 1000 gold stashed up at all times in case of emergency

When two or more manages to somehow get past all the Spring Traps, immediately place a Blackhole Trap at the highlighted square. If some get out alive and your Harpoon Traps are still in cooldown, upgrade the Blackhole Trap to refresh its cooldown. Sell and rebuild to refresh cooldown if you need to.

Wave 5

Finally noticed that I still have not upgraded the Spring Trap to level 4

Wave 6 (Final Wave)

Add two Inferno Traps but always be sure to save enough gold to upgrade or rebuild the Blackhole Trap on the left

Didnt notice it but I just happened to pause it at a kill streak of 999 lol.

Last few stragglers

That's it! Second clear to make this guide got me so much extra Exp that I levelled up from 54 to 62. Enjoy.

[Double Dark]

Lol. That's 1/3 of my XP from a single level.

Btw, you should add the English tag to the guide. It makes it a lot easier to find.

[Shingeki no Potato]

@Adam someone else has already made a video for that: https://youtu.be/uVfvQEI8ORw
Basically u need to make enough creeps drop into the specific hole as sacrifice to appease the devil :3

[Adam Jensen 007]

sorry i am new to the game how do we get to secret level?

[Shingeki no Potato]

@Triggered_Atheist You're welcome :) glad to have been of help!

[Triggered☆Atheist]

Thanks a lot, bro! I beat the level already with another really fun strategy with the same harpoon traps, but it had freeze traps covering the entire enterance and black hole traps that help the line of push traps near the edge, and no spike traps. Darts and a single sawblade were used to help at the very end. It won the level, but had some leaks. I think I lost 5 or so lives. Anyway, thanks. This got me quite a bit more experience, too!