This guide goes into detail on the times you should upgrade walls and buy certain upgrades. It is meant to delay upgrading walls and buying upgrades for as long as possible so that you can maximize your early game produce while also providing a reliable and consistant way to play. It is important to know that this is my personal solo strategy that I use when soloing nightmare, air and invis so it will work on any and all other difficulties. (This guide has been updated for 6.44 )
When to upgrade silver walls into gold walls - 46 minute mark on game timer. ( Since the first tier of armor costs 1 lumber and the first tier of wall health only costs 3 lumber in 6.43 you should buy them either as soon as you make a command center or at the 45 minute mark ) If you don't have a stall and you're getting a lot of mobs then you will probably need to upgrade silver walls to gold a little faster but as long as your getting gold walls around the 46 minute mark you should be good
Upgrade your repair mechanics to engineers ( 2 lumber repair mechanics ) at 45 minute mark ( If you haven't already build an armor aura statue and begin the process of upgrading it, the first gold that you get should go to upgrading your armor aura statue to 1 gold 25 lumber )
Next wait until Absyall grak enters the fortress, this is the time in the game when you want to get repair bots for your walls.
Upgrade Gold walls to Obsidian walls when Cerberus enters the fortress.
around the 38 minute mark upgrade your wall armor to the 2nd tier
When pheng enters the fortress you want to upgrade obsidian walls to onyx and upgrade your repair bots ( not mechanics ) Additionally, you should upgrade your armor to the 3rd tier and wall health to the 2nd and 3rd tier.
Next you're going to be waiting for dreadbane khan to enter the fortress before you do any of your armor upgrades but somewhere in between you should get advanced repairing so that you can upgrade your repair mechanics to the 10 lumber mechanics. When Dreadbane khan enters the fortress upgrade your mechanics, and upgrade your walls from onyx to granite. You should also upgrade your armor and health to the 4th tier. ( The dreadbane khan timer is also a good time to get a food upgrade and upgrade your mercenary tent to the 3rd tier so that you can have wizards for LT stealer )
In preparation for LT stealer you should be focusing on produce as much as you can because you will need to have strong defenses to kill him. If you don't already have them get about 3 wizards and a medic ( You want a medic so that your wizards aren't killed from LT stealers splash ) to use on LT stealer. Also try and upgrade your wall armor at least 2 more times before LT stealer spawns
When LT stealer enters the fortress upgrade granite walls ( You can kill LT stealer with granite walls if you have too, if it means the difference between you getting better armor or saving for a rainbow staff ) to dark walls and upgrade your mechanics to 20 lumber mechanics and upgrade your repair bots as much as you see fit. At this time you should build a 2nd command center ( and later a 3rd command center ) so that you can be upgrading wall armor and wall health non stop and have another command center for other upgrades. At this point in the game your goal should be to have enough lumber produce to be buying armor and health upgrades non stop because these are the kind of defenses you're going to need in order to survive comfortably.
Here are a few tips for walling/upgrading past stealer though. If lumber is an issue and you are starved for lumber armor needs to be your #1 priority! Even if lumber is an issue past this point in the game you can't wait anymore than a minute or two to upgrade armor otherwise your walls won't have enough to survive the end game bosses( Ideally you want to be upgrading wall and health nonstop with 2 command centers). Because you will need at least 18-20 armor level upgrades before Trojan.
Assuming you are upgrading to dark walls when LT. stealer spawns you don't want to wait anymore than 4-5 minutes to upgrade your walls meaning if you have dark walls at stealer you want to upgrade from dark to light at around the 23 minute mark in the game. Light to diamond at the 18. Diamond to Mithril at the 13 minute mark. Mithril to gods seal around the 8-10 minute mark and cursed walls with 3-5 minutes remaining. YOU DO NOT NEED the final tier of wall that requires futuristic technology in order to survive Trojan/Narjot/Command behemoth if your armor is high enough ( I feel as if this is mainly for endless waves ) . Not only is the upgrade expensive but the walls are terribly expensive as well and I find that they are not worth the resources at all. Instead spend these resources on a cursed larceny if possible.
This guide isn't perfect and it doesn't cover certain non wall armor/wall health upgrades but this is the strategy that I follow every game when I solo nightmare air, invis. Hopefully this gives people a good idea on what it takes to kill LT stealer ( I kill LT stealer by buying an elemental blasting staff and using wizards ) But this strategy allows me to maximize my early game produce by being able to focus on lumber and not having to worry about my defenses by knowing how late I can wait to upgrade them.
If you have any questions/comments about this strategy guide leave them below.