What Gensokyo Defenders is
Gensokyo Defenders mixes shoot-em-up action with tower defense. You pick from almost 20 characters from the Touhou Project and hold your ground against waves of fairies. Each character brings their own powers, and you back those up with a selection of Traps that freeze, fry, and blow enemies away. Spell Cards are part of your toolkit too, so a round comes down to where you place your Traps and how you spend your abilities while you're shooting. It's a fan game, so expect the Touhou flavor throughout.
Playing it with friends
The game has online co-op, and you can team up with a friend to take on the tougher stages that are hard to clear alone. Coordinating matters here: one of you can lean on Traps to control the paths while the other pushes damage, or you can split your characters' strengths across the map. Beyond co-op there's online PvP, so if your group would rather fight each other than the fairies, that option is on the table.
Before you start
It's tagged as both Action and Strategy, and that's accurate — you're aiming and dodging in real time while also deciding where your defenses go. If your group only wants to shoot, the tower defense layer will still demand attention; if you only want to plan, you'll still need to handle bullet hell pressure yourself. There's a singleplayer option too, so anyone who wants to learn the stages and character kits alone can do that before joining the others online.