I liked this (v3.0 at the moment) a lot so I wrote my thoughts out, hope it helps!
Sounds, Music, Art design all solid, not too much more to say on them in the general sense.
Nice simple fun gameplay loop a sort of rogue-like reverse tower defense, only beat two so far both taking a bit of trouble until I realized how good range was for getting XP and that XP doesn't scale with damage and going back to beat the first one in under 15 rounds. Edit: Came back and edited this before posting after finishing the third rescue.
I'd recommend tying XP to damage instead of hits, because if you upgrade damage early on you're kinda making it harder on yourself because you'll get less XP.
Dunno if it's fixed yet but I think the attack 'dead' enemy might be when a unit has two enemies at the exact same range so it ends up attacking straight ahead or maybe at the last valid target because it can't decide between them? Or it might be as others have said and it just can only shoot up so it gets stuck.
Oh a UI change that might be nice is some way to select all lanes and spawn 3 of whatever I have selected at once. I'd love to be able to send out more synchronized attack waves.
There's a lot of potential for future expansion on the concept if you want to make a bigger game to sell. More lanes, More enemies, maybe multiple maid rescues in the same map, maybe future mode of protecting with your own defensive monsters while sending attackers. Lots of potential. Enemies that heal/buff others, or jump between lanes (haven't finished it yet so maybe they already exist). Maps with different time limits, or maybe a way to get more time during a round? Different round limits for bonus scenes? Secret/Bonus level if you get all the other ones done in time or maybe just a hard mode?
For the maids, my only complaint is to have some more body shape/size variety, (at least in a hypothetical future bigger sequel/spiritual successor, although that might be more work sprite/animation wise?) Shorter/taller/thinner, ectera. Although from the DX image that might be already changed in DX version! Having more than one maid in a scene too?
Dunno if DX does it but maybe have the maids say something if you click on them during the round? Little neat idea.
I don't see the game reminding you of the 15-round bonus really after the first mention but I might be missing it? If there isn't another reminder you could add in a line in the normal rescue scene for ones 16+ mentioning something like "If only it took 15 rounds or faster to have more time together." or something like that as a in-game reminder? Maybe there's a better way to do it? Edit: Oh it's in the question mark by the pause button on the map. I wonder how many people never click on it like I did until just now... oops. I guess I only checked the main menu looking for a tutorial/help/guide option. Maybe put the question mark as [explanation] or [guide] on the main menu too?
A Bestiary of stats for the enemies (maybe after rescuing that maid or maybe before to help players plan?) would be nice too, maybe a brief description/name for them too? (Screen Shots show that DX has a bit of that so that's good! Still a Bestiary on the main menu too could be nice (Ally/Enemy) variants noted?