Stronghold HD, the remastered version of the classic Real-Time Strategy game of yesteryear is currently Free on Good Old Games. I remember playing the original version about 10 years ago.
It is kind of like Age of Empires in that you have to collect resources, and build an army. But it has city-management too, well, technically it is a “castle builder”, which means you have to keep civilians happy, manage an inventory of weapons, etc.
The AI in Stronghold HD is very challenging, and sometimes it would be so difficult to play the game, that you will lose the campaign frequently. It is clearly not for casual gamers. But it is fun nevertheless. There is a an economic campaign, which has no combat, and is perhaps the easiest. A freebuilder mode, for peaceful gameplay, a "Siege that" challenge mode and a single-player story campaign featuring 21 missions.
To get Stronghold HD for free, go to GOG.com. and login to your website. On the homepage, you will see a banner, which says Stronghold HD and has a "Get for free" button. Click on it and it will be added to your library permanently.
And like all games on GOG, Stronghold HD is also DRM-Free.