I bought this instructions for my son's 7th birthday gift, as he requested.  He is not a big fan of LEGO but really likes GBC videos. He made a few small GBC modules by watching examples from GBC promo videos before, but it was the first time for him to build a GBC module by following building instructions.  With some short breaks, my son took about 8 hours to build the module himself. For him, this was neither too difficult nor too easy, neither too large nor small, but there were a few exciting challenges and lots of fun to build and play.  The instructions were clear and detailed. Timing mechanisms, geneva mechanism, and arms of the caroucel might be more challenging, but they were actually not, because of the detailed "Fine Tuning" section.  On the other hand, the geneva mechanism snagged every round because a large bevel gear connected to a weapon barrel  (#6 on Page 18)  and a small bevel gear under the ratchet mechanism (#13 on Page 24) didn't mesh properly. To solve it, axles/axle-pegs connecting a weapon barrel and shell 3x1x2 technic beams needed to be adjusted by a little poking and prodding, about 0.5 mm or so (Page 19). It was tricky because what he needed to adjust was not the bevel gears themselves but other parts. When my son solved it, he was really excited, looks super happy and proud.  The LEGO Technic 42120 set is a retired product in our country, but I was fortunately able to purchase the 42120 set. Like other PV-Production's instructions, it was really awesome that all the parts came from a single existing LEGO set and we did not have to gather parts up from here and there.  Only if the instruction was printable. It seems that books/prints are still easier for this age group of children to follow instructions. It feels like more than just STEAM learning. It was a big hit and still is. My son wants more PV-Productions' building instructions, and I'd definitely buy.