I wouldn’t call the game ‘extremely high difficulty’, it even has some easier levels early on (at least when I played it a couple of years ago). I’m not a regular tower-defense or sim game player and I was able to complete Serpulo. It can be a challenging puzzle at times, but it’s not a game I’d feel a need to warn people about difficulty-wise.
Disclaimer: this game may be addictive for some individuals.
Seconding (although I have a tendency to marathon the campaign of any game I think is excellent). No need for predatory tricks like loots, this is just a damn fun game.
It’s very weird for a FOSS enthusiast not to advertise one of the best open-source games of all time so here I am trying to make it spoken about again.
IIRC I found it in a ‘top 100 FOSS games’ list because it was one of the first which wasn’t an open-sourced cloning of an existing game. No disrespect for clones and adaptations at all, but it’s extra special to see original softwares so good that even people who don’t care about FOSSness would use them.
Apparently it was popular in US prisons until it was banned.