Treehouse Builder

Treehouse Builder is a cosy physics building game set in one huge old oak, seen from the side under a leafy canopy, with birds hopping along the branches and a swing hanging from a low twig. Pick a piece from the tray, planks for platforms, posts, diagonal braces, rope ladders, a slide, a rope bridge, walls, a window, a pitched roof, a pulley bucket, lanterns and flags, and drag it out from a knot on a branch or from a piece you already placed. A ghost shows where it will land, green when it fits and red when it does not.

Press Test and a parade of cuddly visitors, teddy bears in four colours and one rather heavy goat, walks in, climbs the bark or your rope ladder and stands on the platforms, and then the wind picks up. The structure is worked out by a real, step-by-step physics simulation that gives the same result on every device: planks need support at both ends or a brace underneath, loads travel down to the branches, and an overloaded plank bends, creaks and finally droops, tipping its visitors gently into the piles of leaves below. Nothing breaks and nobody gets hurt, and Back puts your build back exactly as it was. Forty brain-teasing challenges on the same oak ask you to reach a high branch, hold six visitors on one deck, join two platforms with a rope bridge or build a room with a roof, all within a wood budget, with three stars for the goal, for staying under budget and for not a single droop.

Free build has no budget and no goal, so you can relax and build the treehouse you always wanted, then switch to decorating with lanterns, bunting and eight paint colours. Keep up to twenty treehouses in your browser and press Photo to save a picture of your favourite. Undo and redo go fifty steps back, there is an eraser, R turns a piece, and the zoom buttons or a pinch bring you close to the knots. It works with a mouse, a finger or the keyboard.