I would pick more dynamic poses for the characters (unique for each, not just a mirrored version) ... It might help to not think of them as poses, but as reactions to a threat in the story the scene is telling. A dark background will suit the mood better.

Loving the sprites and animation! The shotgun blast stands out as a little odd to me, being perfectly circular. It doesn't seem to match the zombie's dynamic death animation. Minor crit, though, looking great!

The lines on the floor tiles align with the edges of both types of raised platforms, and flatten the image. I think the blue edges should be a tad brighter (especially along their outermost edge, since the existing colors seem to imply rim lighting) and thinner. Right now they almost look the same size as the floor tiles, which makes the grid much more obvious. I would also darken the floor and put some small random lights on them to reinforce the setting. Might be worthwhile to turn the crates at angles, adding visual interest and breaking up the grid.

Sprite designs look fun, I like the variety in their silhouettes.

Regarding the plant biting animation -- what if it opened its "mouth" during the rearing/anticipation frames? I think that would better communicate the impending danger.

If you look at TF2 characters, a lot of care was taken to give them unique silhouettes so they would be instantly distinguishable in-game. Because the colors are unified to show team affiliation, the shapes become even more important for identifying unit type. Currently, the king and the knight share too much -- same approximate size, same head atop an upside-down U body. I recommend exploring the contrast between sharp angles and curves to highlight the differences between these units. Best of luck!