This was the perfect Christmas present for my twin 5 year olds! I ordered the boxes based on customer reviews and my sister in law as I bought these for my nephews many years ago. The arrows point to the top hole so you can orient the structure correctly. great for adults but a bit difficult for 5 year olds to follow and understand. The construction is solid when built well, but the balls get heavy if you try to economize on construction. We had no problem keeping 3 or 4 flat sheets on our forts. It was a great activity for all of us as we were confined during the brutally cold Iowa winter! Mom loves building new projects every day and kids spend hours disappearing into imaginary rockets, castles, igloos, tunnel systems and fortresses! (They also have a Facebook page for inspiration and feedback)