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Which map are you playing on? Different maps have different difficulties, and sometimes (as you say) it's possible to warp through the tiers very quickly. Other maps are much more difficult: today, testing in caves, I was having trouble setting up reliable steel production, even by wave 8, as the enemies were harassing my production. A few updates ago, when I was playtesting in fortress, I was producing dirium by that time.

It also depends on how the ores generate around your base. Sometimes you get lucky, other times you don't. Still, you're right: very often, it takes too much little time to progress, and some of the turrets are quite useless. I will look into this when I can.

As for deletion, I agree, and that has been fixed in the 3.1 update.