If you find pollution is not a challenge, try a deathworld run. If you can already beat a normal deathworld - turn down trees/moisture, turn up pollution diffusion ratio; then you can add rampant, armored biters, bob's enemies, etc. I recommend the infinite ore patches mod as well, you will be defending the starter patch for a veeery long time.
My favourite recent challenge was "New Game +", you start with all tech (perhaps except damage upgrades) unlocked, but biters also start at 1.0 evolution. You technically don't need any science to launch a rocket, but you still have to figure out how to build a mall (and enough defences) before triggering a single attack.
It really is, isn't it. You're incentivized to protect every last thing, not because of some arbitrary points system, but because they're all part of a big, interconnected system that needs to work in order to survive.
In my book, Factorio is up there at the top of best games of all time.
My favourite recent challenge was "New Game +", you start with all tech (perhaps except damage upgrades) unlocked, but biters also start at 1.0 evolution. You technically don't need any science to launch a rocket, but you still have to figure out how to build a mall (and enough defences) before triggering a single attack.
Factorio is an excellent tower defense game!