Two myths about the wall and why some players should get the wall early
This post is for eHP players who do not have a WHR and farm using blender on t6-t9 wave ~5000.
If the above describes you, you should ignore 99% of what you've read about the wall and get it much earlier than most well-written guides (like this one and this one) on the subject say.
Myth #1 about the wall: Because of the heat-up mechanic where enemies do 4% more damage with each hit, having an underdeveloped wall will mean you get one-shotted when the wall falls, and your run will end early.
While this makes sense when you first read it, for almost all players it is just not true. The reason for this is recovery packages. When your wall is up, you will collect recovery packages and they will remain essentially untouched (only vampires reduce them) as along as your wall stands. Most players who farm t6+ will have their max recovery gold boxed in the first 1-2k waves of a run and so they will have 16.5x of their base health when the wall falls ready to absorb enemy hits. The wall health on the other hand will not be close to even 1x of base health, but even if it were maxed at say 3x base health, the next hit that one-shots the wall will be 3.12x health and still easily absorbed by the packages. Even if there are 40 enemies surrounding your wall before it falls, that means they will each be doing 0.075x of your wall health in damage when it falls, and packages will easily absorb the damage.
When I got the wall and had zero research done on it I took it on my usual t8 farming run and over and over I watched this play out. The wall fell and my recovery packages absorbed the enemies. My run ended in the exact wave range it always did before getting the wall. There was zero negative effect to having an underdeveloped wall due to my packages tanking the hits when the wall fell.
Myth #2 about the wall: You need to be making 200B or 500B per day to be able to afford the labs to make the wall work.
This may be true for those that have and use a nice juicy WHR, but if you don't have one then it takes barely anything before the wall surpasses your blender build.
On my first run where the wall held up I had level 2 wall thorns, level 2 wall regen, and level 1 wall wealth. That is 154B worth of research and my wall lasted the same number of runs that my pre-wall blender build lasted.
On my second run I had level 3 wall thorns, level 3 wall regen, and level 7 wall health. My run went 700 waves further than my previous blender record. Coins and cells were far greater than normal as well. That is all for just under 250B (total, not per day) and now my wall is better than my tower was before I unlocked it.
For context, my other relevant stats were:
7* common cards
6* rare cards
Cards used: EB, WA, PC\*, regen, extra orb, coins, AS, crit coin, ED, FU, WS, health, cash
Legendary NMP with 5% def, 40% regen, and -2s shockwave freq
DW health research level 13 (825%)
Health research level 34
Health regen research level 30
Health WS level 3800
Health regen WS level 2800
\*Once I noticed the boss was in range of my orbs due to enemy build-up around the tower, I unlocked 2 levels of orb boss hit and with more levels of this I might unequip the plasma cannon.
;tldr: If you don't have a WHR and run blender/eHP all you need to make the wall useful is 250B to get thorns/regen to level 3 and a few levels of wall health for good measure.
In case you want confirmation, here is another post from 1 month ago of a player who got thorns/regen to level 3 and had a record run. Another player whose wall was good after level 3 of regen. And another post that identifies level 3-4 of regen as when the wall is better than a tower with no wall.