I thought this game was well known but I haven’t seen almost any mentions of it recently. 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.
Disclaimer: this game may be addictive for some individuals. Player discretion is advised. If you notice any symptoms of addiction while and/or after playing the game, stop playing immediately and consult with your doctor. Untreated gaming addiction may result in severe consequences such as digestive disorders, social behavioral disorders, loss of job, and depression.
Mindustry is much closer to the tower defense side of it.
Or maybe the RTS side.
There are even attack modes, where you build up an army to take on a fixed emplacement of enemy units.