Well, I managed to beat the game by placing one portal on each soldier. Considering how squishy the skellingtons are, that was the only way I could find. Neat idea with a certain degree of Game Jam Jank.
And this is probably a lucky accident, but: The sounds of the skeletons walking, the gunfire and the skellies dying actually had a really nice rhythm to it. It reminded me a lot of the sound design in Heroes of Might & Magic 2, where the environmental SFX matched the music in a very satisfying way. If you're going to make another reverse TD in the future, maybe you can try and have the attackers and defenders match the music. It would make for really engrossing gamefeel, I think.