Not all those who wander are lost
IF not too late, a very good construction method is to bore a hole down, top to bottom, on each side of each corner, and pound in a solid steel rod (re-rod will work and is cheap). The interconnections of layer to layer are good, but this method above is a lot stronger. You can do this along each wall too, but it's more work and might not be necessary. I know it's something I'd do, especially if from scratch. This will help keep bowing from happening to some extent. Research 'buttress' for above ground ponds and see if that design is something appealing. This may all seem like overkill (it does to me even as I write it!) but if you start reading about bowing, leaking, splitting, rotting, etc of such wood above ground ponds, you may not think it's overkill at all!My pond is 6ft 6” x 4ft 6” the timber is 3an half inch x 2an half inch an I’m using 6” screw bolts down through the corners and thr the sides at the corners recessed That go nearly through the wood and 5” screws in the tier sides
Good on you for an open mind and NOT wanting to do it again later on!