If you love classic 2D RTS games like Warcraft 2 and other similar titles, Wyrmsun is one you should take a look at because not only is it free and open source but it's seeing major upgrades too.
Their original plan to move to Godot Engine didn't work out, so they decided to stick with the classic Stratagus engine which it was originally based on. Instead, they continued enhancing what's was possible with what they already had including some substantial tech improvements including a 2x scaling option, improved pathfinding performance, parallel sound loading to improve loading times, mini-map zooming and more.
Lots of gameplay enhancements were also added in Wyrmsun 4.0 like a smarter AI that will now try to aid its buildings under attack, the AI will try to actually complete its quests and it only declares war during scenarios after 20 minutes so main quests go a bit smoother.
A number of art updates also went into the 4.0 release like Germanic buildings getting snowy variations, Copper, Silver, Gold, Iron, Mithril and Coal Deposits and Mines now have snowy variations, building town halls over settlement sites now possesses two additional (generic) construction frames and more snowy variations to other buildings. There's also a new Latin Dock, Elven Town Hall, Stronghold, Farm and Barracks buildings.
Pretty huge upgrade overall for a great FOSS RTS.
You can grab it free and keep it up to date on Steam with more info and source code links on the official site. Currently the 4.0 update on Steam has only been updated for Windows, the Linux update is to come a bit later. You can grab it yourself on Linux from GitHub until the Steam package is up to date again. The developer has a few simple paid DLC or donations to support their work.