It was determined that bracing on the long sides was needed. Much better, now! The rope ladder from the tree was moved to the fort, though it is less than satisfactory. The contractor is now thinking that creating a permanent ladder on the end would be best, with a thick knotted rope to climb at the trap-door entrance.
I thought this might be the sort of project that would be completed at some point. I see now that I was quite wrong. Apparently the beauty of building a rather simple, open-ended play structure in your own backyard is that construction is not meant to ever be entirely done. There are infinite improvements, additions, re-builds, and de-structions to ponder and pursue.
Ah, well... as long as I don't lose my laundry-line hook, it's all good with me!