I used to tape every seam on my forts, now I cut flaps and fold them into slots, and the thing held up through a 2 hour backyard war with my nephews

Man, it's a cardboard box fort, not a load bearing wall. Your nephews are gonna knock it down either way, slotted flap or taped seam. I've built plenty of these things with my kid and honestly the tape vs fold debate matters way less than where you put the door and whether you have a good stash of pillows inside. Two hours is a solid run though, I'll give you that. But corners where three panels meet? Usually I just shove a wadded up sock or a sweatshirt in there and call it done. The fort doesn't need to survive a hurricane, it needs to survive until snack time.