Kids love playing in forts, so when a couple of big plywood packing crates were being disposed of from work, I snaffled them and with a bit of modification converted one into a castle with drawbridge for inside play, and another into a playhouse for the garden.

Step 1: Materials and Tools

I used a cordless drill and a jigsaw for this build, plus layout tools (measuring tape, pencils, square).
Materials were principally two large plywood packing crates, plus I extracted some rough-sawn planks from a heavy-duty pallet. I had a few 2x4s and some 2x2 cedar leftover from a deck rebuild. I also bought a roll of building paper and enough cedar shakes to cover the roof for the playhouse. We used a carpet offcut and some paint and polyurethane for finishing, plus some fence hardware (hinges and latches). Overall cost for the two forts was about $50.

Step 2: Playhouse

I mitred together a rectangle of 2x4s whose internal dimensions were the same as the box (this gave the roof a slight overhang). Over this I framed 4 rafters with a 38° pitch, roofed it with ply from the top of the box, stapled building paper on then shingled it with cedar shakes. The door and windows used fence hardware (hinges and latches; the latch can be opened from inside with a drawstring). The windows fold down; fortunately, the roof overhangs enough that it tolerated the windows being left open a lot. The exterior got painted with polyurethane and the interior was painted white.
A porch, curtains, carpeting and furniture were later additions.

Step 3: Castle

I cut out the drawbridge and arrow slits with a jigsaw, and recycled a big pallet for the boards that formed the crenelations and front tower. I used a handsaw to trim the top boards to accommodate the tower and crenelations. Note that the drawbridge can't be pulled back into the castle because the boards are cut bigger all around than the plywood (see the second photo for detail). It's all held together with wood screws (drilled and countersunk from the inside) and construction adhesive. We used an offcut of carpet to cover the floor. We added a cloth drawbridge pull (see third photo) so the kids could close the drawbridge from inside.

I haven't given any dimensions, since these will depend entirely on the size of the box you use! Note though that using an odd number of boards for the front tower makes the crenelations work nicely, and reduces the amount of cutting you have to do for the drawbridge.

The castle got repainted several times by the kids, at least as much from graffiti buildup as desire for change. The final paintjob had a pink interior and ivy growing all over the outside (thanks to a cardboard stencil and some poster paint).