I'm a little fuzzy on how spell traps work with being triggered, so let me set up an example:
A Fireball trap. It hits a 20ft radius, all targets in that area. Triggered via proximity of the Alarm spell, which based both on the spell description and the spell effect is also the 20ft radius area. I assume when placing a trap of this nature, you still pick a square or intersection depending on the case, which would be the same point for both spells, and in this case have the same circle of effect.
So does the fireball go off when the first person steps within that 20ft radius circle? If so, then assuming the party is moving one at a time in a round because the players take turns and also assuming it wasn't detected, then only the first person would take any damage. Am I understanding this correctly? If so that seems like a waste.
Please help me understand how this is meant to work. To me it would make more sense to have a smaller radius trigger to get the whole party into the area before the trap detonates in the case of an AoE effect like fireball. I feel like I am either missing something or not understanding the system correctly.