Yep, melee units against ranged units (the thief counts as a ranged unit)

Hmm I don't think it's THAT hard
I never even used all the monsters, mu and shade seem pretty useless...
Just be sure who's attacking and when, attack floors with appropriate monsters (+ healer sometimes), lots of support floors if there are multiple enemies, and put puppet theaters on the bottom if you want to weaken adventurers or would like to buy some time for monsters above to heal.
No problems as of yet even with the side-missions, I've got all the NP-raising rewards (which are really recomennded or else you'll be f#&ked later on )

I love the Tower Defense genre, and Square Enix took it literally
Really great stuff, I am at the 4th chapter now, sometimes it takes a few tries until you find the right strategy for the level. I'll look into the DLC as well, I hope it contains more levels!

Damn, I was hoping for a theme hospital experience, too!
Now I have to install and configure dosbox and install TH to play!