Steel furnances recipe and technolgy is changed.
Instead of
tech: steel->steelfurnace; (steel for furnace )
its now
tech: steelfurnace->steel (stonebricks for furnace)
Steel and silicon can only be produced in steelfurnace
(the reason why I left the name "steelfurnace" is because it SMELTS steel. So it still makes sense

)
It might be a legitimate argument to say that this skips stone furnaces too fast, but for me the slow smelting of stone furnaces in the beginning felt redunant. The recipe for the new steel furnace is not too cheap(25 stone BRICKS and 8 iron) so stone is a more required resource and it will take some time to upgrade your furnaces.
Did you consider making two gun turrets, i.e. keep the current cheap early-game gun turret, and add a beefed up gun turret that also requires e.g. concrete?
Actually I wanted to avoid additional buildings as much as possible to keep the mod slim, but I will consider this. Espacilly new usage of concrete for turrets or walls is something I like. One point I try to make is that in my opinion a different building also needs to looks different(for "readability"). I want the mod to keep the looks of the vanilla version as much as possible and if I introduce a new gunturret I have to recolour it somehow. My idea is that new buildings, if I introduce them, can be distinguished by the player, but still look like its the normal game.
When I first tested it, I didnt felt like the steelcost of the first gun turrets was a big hinderance. In my opinion its totally fine this way.
There is an interesting mechanic: Gun turrets scale better in the long turn, because of technology and laser and gun turrets have similar DPS in lategame(gun turrent a little bit more, because of lower range).
Also I changed the balance of bullets, the early game of pistol and sub machine gun is stronger (3 instead of ~7? hits per enemy), so you are not too reliant on gun turrets for the first waves(as long as you have ammo ofc ^^)
Another interesting thing with bullets is the "assault rifle"(lategame):
Submachinegun: 15/s
Assault rifle: 5/s, +200% damage, higher range
Actually this is an indirect DPS buff(because of enemy armor) and makes "players-shot" bullets equally good than "gun-turret-shot" bullets. (gun turrets have so much technology bonus, so the +200% is a compensation for the tech bonus).
This are the thoughts behind the balancing
As I wrote in the first post I will rebalance combat in the future, but now I am testing what I created and I will finetune the mod on the run.
However, if I rebalance combat I might introduce Tier 2 turrets, but my main focus is on making flame thrower stronger(maybe with enemy negative fire reistance), spawner more durable, granades stronger etc (=weaker weapons viable and niche application)