As a developer, I'd never thought I'd like to solve for getting off the planet, fighting monsters, building trains and tracks while trying not to pollute too much.
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!
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.