Gameplay you have so far is already pretty fun. Very cute girls. Gattling gun is definitely my favorite weapon. Has the best audio / visual feedback of all weapons so far.
When firing a weapon for the first time there's a 1 frame stutter. If I had to guess it's Godot loading the assets / shaders for the bullet trails. If I'm right you should try preloading them on game start if possible.
Sometimes it can be hard to tell where you're getting hit from. Game could definitely benefit from some sort of damage indicator like picrel (where the red shape indicates the relative direction of impact).
I'm also not too sure what fortress mode does. Does it just slow time down a bit? The naming makes it sound like it improves defenses but I still got mowed down pretty quick with it on. I could see the noise it makes when you turn it on annoying when players use it frequently.
Other than that I like what I see so far. Looking forward to further updates. Keep it up!