i like your game quite a lot!
it's simple yet fun.
i do have some minor issues though...
first, there is some lag, especially in later levels with more units. looks to me like the game stops a few frames whenever a unit gets hit, which happens a lot. while i do use a pretty low-end machine, lagging on this kind of game should not happen.
second, there seems to be some minor input delay during snake movement. i already figured out that the snake seems to only accepts inputs the frame it completes the moving to a new square and that seems to be the mayority of the issue. i think a change from checking only on that frame to queueing up an input or maybe even two inputs would make moving the snake feel a lot better.
third, sorting units gets kinda difficult once you hit multiple rows. the menu sometimes collapses whenever i move over the border of a unit portrait. i think they don't quite line up borderless.
...could also be a fullscreen issue. if you press alt+enter in this kind of gamemaker programm, you can make the window fullscreen even if that is not actually intended. i use that a lot and some games show some strange issues only in this forced fullscreen mode, so i thought that may also apply here.
forth, the difficulty scales a bit too steep in the late levels. i mean, i didn't even get that golem to beyond halfway to the exit on the most direct path in 1-3.
i think an upgrade system and maybe the ability to sell units would improve the game quite a bit.
i don't mind if you restrict that to only between floor transitions, but it definitely should be added.
so, with that wall of text out of the way, i say it again, i like your game quite a lot and those issues are only minor.