Nowadays, basically every multiplayer game can be an eSport. But from the beginning, Coregrounds has been made with eSports in mind. It's a multiplayer tower defense game that's now only fun to play, but also to watch.
In 2015, we've proven the eSports nature of Coregrounds with a tiny little championship. There's a recap post which contains links to the match history and the VODs for all matches played. This championship wasn't a huge spectacle, but it has proven that Coregrounds could one day be an exciting eSport!
But how does that work? What makes a game entertaining to watch? Skill does: both its presence and absence and everything in between. Coregrounds is so entertaining because even though the basic gameplay is very easy to learn, there are so incredibly many little things which can make a huge difference. And using all these tiny little details to your advantage is what makes skill on the Coregrounds.
Even before a match starts, you can turn the game your way: by drafting a build with units that work well together or make up for each other's weaknesses or by picking units that are particularly strong against the ones your opponent has picked, you gain an advantage before you even start to play. And ingame there's even more room for doing it right or doing it wrong. Timing, reaction, planning ahead, game sense - there's just not enough time to do everything right. Skill decides who does it better.
All units, abilities and gameplay mechanics are carefully balanced to make all of this possible: and even though there always will be units which are stronger than others, the main design goal is and always will be fairness. If the units' strengths wouldn't be balanced, there'd be no point in talking about skill. The meta will be constantly changing, especially as we add new units and abilities regulary, but the balancing is probably one of most important part of making Coregrounds work as an eSport.