It all depends on how you look at it, I guess. If the watchtower is looking into the distance, then creatures can naturally be seen. I get that.
I guess an explanation for spells could refer to instructions/incantations/prerequisites to cast the spell, seen by the watchtower as the enemy prepares to make the necessary changes to its assault.
Admittedly, creatures only does make sense. However, watchtowers cant see burrowed things, either. Nor could they miss large structures like 'sanctuary' or 'titan'. Why are permanents excluded?
What is your thematic view of the watchtower?