Got this message when adding a tier 11 and 12 as suggested possible with the settings.lua edit:

The mod Schall Endgame Evolution caused a non-recoverable error.
Please report this error to the mod author.

Error while running event SchallEndgameEvolution::on_gui_click (ID 1)
Unknown sprite "sprite-Schall-tier-12"
stack traceback:
SchallEndgameEvolution/control.lua:177: in function 'update_gui_evo'
SchallEndgameEvolution/control.lua:381: in function <SchallEndgameEvolution/control.lua:374>
stack traceback:
[C]: in function 'newindex'
__SchallEndgameEvolution/control.lua:177: in function 'update_gui_evo'
SchallEndgameEvolution/control.lua:381: in function <SchallEndgameEvolution/control.lua:374>

Not sure if I did something wrong OR if by "adding extra tiers" you meant the difficulty options?

Thanks for looking Schall!

Oh, I hardcoded max tier 10 in sprites, because I have not prepared the tier sprite icons under /graphics/gui/.
I did not do the "auto-detection" of max tier setting (as step 1 below), because any missing icons will prevent game from loading.

For the moment, you have to do the followings as temporary solution:
A. For sprites "sprite-Schall-tier-*", edit /data.lua line 4 as:

for tier = 1, tier_max,1 do

Then add the following line BEFORE it:

local tier_max = settings.startup["endgameevolution-alien-tier-max"].value

B. Go to folder /graphics/gui/, clone some icons and name as tier-11.png and tier-12.png. They only affect the GUI icon on top left. So it is only for visual look, so it does not matter if it is the wrong or same icon.

In the next update, I will prepare some extra tier sprites icons, if crazy players (like you) :-P really want to try extra high tier enemies.

Thanks for the feedback!
Right now I also tweaked it where the X level biters have 9,600,000hp for the biter and 4,800,000hp for the spitter lol.

So many layers of defence, walls, mines, dragon's teeth, flamethrowers... It's an amazing challenge for late game so thankyou for this!

Haha! Sounds like it is way beyond my imagination. :-D

If you have the time and want to share, you may post some screenshots of your defenses here. Guess it would be quite interesting and useful design.

PS: About mines, I am going to release a new mod this week Thursday, about more types of land mines. Though I am not sure whether they have enough firepower for your uses. ;-)

Chants Higher tiers! Higher tiers! Higher tiers!

Right, just released 0.17.7, which includes the sprite icons for extra tiers. (For up to category XX or tier-20 at most.)
No longer needs the temporary solution stated above.

Yet you still have to do one thing in the new version: edit "settings.lua" and add options for values larger than 10 yourself.

I don't trust myself to mess with the actual game code

Feel free to do whatever.

Strictly speaking, it is not game code. Only a setting in the mod.
It is just a single line in settings.lua:

allowed_values = { 4, 5, 6, 7, 8, 9, 10 }

Modifying this line to the following will do:

allowed_values = { 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 }

I have not provided options higher than 10, just because I cannot guarantee the weapons or turrets (at least not from my mods) can handle them well.
I may "officially" provide higher options if I will have some advanced weapons mod being made, but again it is no guarantee.

Because you cannot guarantee weapons will be able to handle them? So? That's one reason people download this is to get over run and really test your defenses. Also there are other mods that can be used with this, like turret shields and double turret range. Plus If you can't handle them you can always just pop over to settings and turn them down right;)
Also as a loving member of this mod I feel I have generated a number of downloads from various reddit posts directing people asking here and I'd really like to ask if you can please hard wire some higher tiers in. I'll make a great reddit video post showing off the biters going ham, I already have a good idea

Alright, you have a point. And I should not hide a feature that this mod is already capable of...

I need some time to think of what to do. Maybe just adding a description text to the option, to "warn" users that options beyond 10 can unleash aliens which are really crazy?

Thanks for you love of this mod! I believe players who found the "weak" vanilla aliens boring, will be impressed your videos though. :-D

Yes! Thank you! Oh definitely, from my reddit posts people have been blown away by just the level 7 and 8 biters!
I'm going to make a battle of kaludathu style vid and make wall look OP but have it ripped apart.

Yes warning should be set and despite what you may think I feel that "warning" will also be a incentive to download.
"What possible monstrosity needs a warning in factorio???" and I hope I'm not asking to much but since we're already up here but for added insanity maybe do something special for level 20's? ;D

Just uploaded 0.17.8 with new options up to tier 20 (category XX).
It maybe better to try the "intermediate" tiers first, before jumping to category XX directly.

I tried it myself, with my own mods Schall Ammo Turrets & Schall Alien Tech. My damage upgrade techs are level 7 in my test save (first infinite tech level researched). The result is my defenses can barely handle category XII spitter, where it takes some long time to kill and some occasional destruction of front turrets. Maybe I can improve the performance a bit with more military research and more long range turrets (cannon turrets, sniper turrets) and force cannon turrets to push back. But I guess category XV would be the absolute maximal tier my mod set can handle.

BTW, instead of installing more OP mods to "prove" how OP those turrets are... I guess it would be even more interesting if stating some "challenge" in some of your posts. Like restricting to a specific mod set of turrets, and see how well they can perform. This would be more comparable and/or challenging, as the "competition" then goes to how well the defense layout is (instead of how OP a turret mod is).

If you have a video or post ready, you can put down a URL here. I am also interested to see how your combat goes.