Coregrounds is not the first multiplayer tower defense game per se, but it's the first one that's played one-on-one on a shared battlefield. There are other PVP tower defense games out there, but they're usually not more than a single player TD where the multiplayer is nothing more than sending creeps to your enemy.
Coregrounds is fundamentally different: in Coregrounds you and your opponent share a battlefield - and this is where it gets interesting: Instead of just defending and sending creeps, you have to manage both offense and defense at the same time. For that purpose, towers can be moved so they can do both. And since both players build their mazes in the same space, the way you build your maze and reacting to your opponent's wall building is also a crucial part of the gameplay. The fact that most units and abilities in Coregrounds can be used both defensively and offensively, as well as the fact that there's dozens of them to choose from in each match is what makes Coregrounds a real multiplayer game and an aspiring eSport.
And apart from the basic gameplay mechanics, there's a lot more: You can upgrade your units for example, which makes your units stronger, but takes time, during which they can't be used. Units also gain experience in battle. This way, player skill decides who'll win a match: cleverly drafting a build of units and abilities, knowing when to fight and when to retreat, when to upgrade and when to farm XP is what makes a good Coregrounds player.
If you want to read more about how the game works, check out the Game Guide. Of course you can also create an account and start your journey on the Coregrounds. There are ingame tips to help you get started, but it definitely makes sense to check out the tutorial video first, it's less than 10 minutes and will give you a head start. Good luck and have fun on the Coregrounds!