Rise of Legions is a "castle siege RTS" built around "short, intense 1v1 matches" that has players trying to smash their opponent's castle before their own castle ends up similarly schmucked. It's based on the old Warcraft 3 mod Castle Fight, if that rings a bell, it's free, and it's playable today on Steam Early Access. 

Players will collect cards to build decks of unique units, taken from across three unique Legions—factions, basically—which can then be turned against human opponents in quickplay matches or PvE "challenges" against AI-controlled bosses. The gameplay options are fairly basic right now, but it looks pretty good and doesn't require a significant time investment to play, as matches will normally only take about ten minutes to complete.  

The developers warned that Rise of Legions "might be a bit rough around the edges at first," but said that all listed features are implemented. In-game purchases of skins and cards are available, and players can redeem their Steam key in the Rise of Legions store for some free currency to get things started. Over the Early Access period, they plan to add more Legions, 2v2 and ranked play, tiered matchmaking, custom multiplayer matches, and new units, skins, and other features. 

If you give it a shot and encounter one of those rough edges, there's a Discord server where you can go for help—or if you just want to hang out and dispense ideas and suggestions, which are also welcome. Rise of Legions is expected to go into full release sometime this summer.