stimulate more production with timers, at least for the first one

[Sworn]

To encourage new people to increase the production, the biter waves was increased by a lot.

So in the first steps of the campaign, before you get started with combat, would be a nice transition to stimulate a fast research, through a good production of science, like "research X tech before Y time, so you are able to defend yourself" and during this same quest give some indications that you need more production, like make the player build 10 labs to research, with some science buffer that won't be enough to do all research.

Then at the point you don't have enough science packs to keep all 10 labs running, the game suggest that you need more production so you can keep all of them working, and plus all that give another warning like "At current rate, the biter will be here before we can finish. So better bump up the production up to 20 labs".

Then after the research rush you unlock some tech, like flame or what ever that make you defense way easier.
Sure oil and all those steps would make it way to hard, so it's just a example, could be anything that helps, the flame itself could be already prepared but without one step that require a tech research to be done, with some finite ammo. That is just to encourage the production not the tower defense with layers of layers of walls, that can be introduced at further in the campaign, after the player already understand the production concept instead of leaving the game afk until the amount X of items are made in very slow rate.

Edit
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Then, in the next mission, before the attack, some chat like "Would be nice to have those flame throwers, but I don't see any fuel for that, gonna need to build some more turrets for now. Better keep up a good production of ammo" that will remind the player of how importante is to keep the production UP and encourage to build more turrets