I'm confused about some aspects of the Antilife Shell spell, which reads as follows:
School abjuration; Level cleric/oracle 6, druid 6, shaman 6; Domain animal 6; Subdomain souls 6
Components V, S, DF Casting Time 1 round
Range 10 ft. Area 10-ft.-radius emanation, centered on you Duration 1 min./level (D) Saving Throw none; Spell Resistance yes
You bring into being a mobile, hemispherical energy field that prevents the entrance of most types of living creatures.
The effect hedges out animals, aberrations, dragons, fey, giants, humanoids, magical beasts, monstrous humanoids, oozes, plants, and vermin, but not constructs, elementals, outsiders, or undead.
This spell may be used only defensively, not aggressively. Forcing an abjuration barrier against creatures that the spell keeps at bay collapses the barrier.
Since the spell has a casting time of 1 round, there's a risk that the enemies you're trying to hedge out will close to melee range before you've finished casting. I'm trying to figure out what the line "Forcing an abjuration barrier against creatures that the spell keeps at bay collapses the barrier." means in this case. Does the barrier immediately fail, or does the caster have the option not to force the barrier into the occupied squares? If the caster backs up, does the barrier prevent enemies from the closing the distance a second time?
This question is inspired by the Kingmaker adventure path, which features an lone caster enemy whose published tactics say to use this spell in a 20 ft. diameter room. RAW, I don't see how the spell can possibly be effective - even if the enemy takes the time to turn invisible first, the odds of there being a space in the room where the spell can be cast without immediately collapsing are essentially nil.