ignite_knight hospitaller 2A square medieval castle on a square island is under siege. All around the castle there is a square moat 10 meters wide. Unfortunately, a group of  raiders have brought footbridges that are only 9.5 meters long. The invaders do not want to fail in their assault.  How can they use their footbridges to invade the castle?

The answer to this puzzle, and 100 others,  can be found in a new kindle ebook called PUZZLED, and is available in the UK here and USA here.

88 comments

  1. Perhaps they can paddle them across.

    Actually if the castle forms the inner side of the moat and rises vertically from the moat (which is how I recall seeing pictures of moats around castles) then I don’t think it can be done.

    I also think that the solution that the puzzle setter has in mind doesn’t take into account the integrity of the outer side of the moat, that is, if it is just dirt and the raiders are being shot at then they would have problems.

    Finally, what about the monsters in the moat?

  2. What they need to do is freeze some urine, and attach these blocks of the frozen urine to either end of the walkways, thereby making them longer.

    But yeah, I got it before finishing reading the question. Assuming a few little things, but yes.

  3. 10-20 seconds after starting to think about it.

    All you have to do is urinate on the… O, it’s been done to death already. Dammit, and I thought I was so original 😉

    I can only think of one solution though. I’m curious about what other solutions you guys came up with.


    http://noamgr.wordpress.com

    1. well – I’m willing to learn… I’m quiet sure the official solution requires 3, but I’m looking forward to the “2-solution” on monday

    1. Yeah but if it’s a moat, the water’s not going to be flowing particularly fast. You lat it on the water, and get your trusty valet (read: pawn) to secure it using rope or shoelaces (as someoen else said). Or just tie his feet to the end of the footbridge, get him to hold on to something at the other end and just use his back as an extension?

      Also, aren’t footbridges really easy to make? You just tie a load of wooden planks together with rope. What’s to stop you taking another footbridge apart and reusing the pieces?

      Am I thinking about this too laterally?

    1. when are the answers to these puzzles ever ‘good’ everyone seems to think they need to overthink the solution.

  4. Again another puzzle that is “too open”, and urine or not the good folks here will think of at least 5 ways of solving it. I can think of a couple instantly – although I’m wondering if the ‘squareness’ is an important factor?

    1. It’s not open in the same way the last one was; there is no need to invoke deus ex machina solutions. That said, I’m laughing like hell at all the urine references this time around! 🙂

      For the classic answer, yes, squareness (or, rather, being a polygon of certain measure) is an important factor. I heard the puzzle before, so I knew that answer immediately. After thinking about it for a couple minutes, I figured out a three plank solution that works just fine on a round moat as well.

  5. I don’t think that any invaders stupid enough not to even do the basic background planning like working out how wide the moat is deserve to succesfully invade the castle so I aint telling them my solution!

  6. It came naturally, got it before I started to read the question 😀 (It was like “So?.. Oh, that’s what’s asked..”) But to check whether it’s really really right I needed a bit more time.

  7. My immediate sub-conscious response was the word:
    Tensegrity.

    Now I have to work out just how to fit that engineering concept into a solution…

  8. I like the idea of the square moat. If it had been a round one the same solution might have worked, but it would depend on the radius, right? For the square one the length of the square does not matter which makes for a better puzzle. Brilliant.

    1. Well if u collect all the urine of your man, and make a catapult out of the bridges. Then shoot is all in the castle maybe you can force the enemy out.

  9. Took me a couple of seconds after finishing the article. I think it may be the two plank solution others are thinking of. However, 8 planks would be better, and extra planks wouldn’t hurt.

  10. The solution “wait for the moat to freeze” reminded me of the following joke:

    Joe heard a rumor that his father, grandfather and great-grandfather had all walked on water on their 21st birthdays.

    So, on his 21st birthday, Joe and his good friend Brian headed out to the lake. “If they did it, I can too!” he insisted.

    When Joe and Brian arrived at the lake, they rented a boat and began paddling. When the got to the middle of the lake, Joe stepped off of the side of the boat… and damn near drowned.

    Furious and somewhat shamed, he and Brian headed for home.

    When Joe arrived back at the family farm, he asked his grandmother for an explanation. “Grandma, why can I not walk on water like my father, and his father, and his father before him?”

    The feeble old grandmother took Joe by the hands, looked into his eyes, and explained, “That’s because your father, grandfather, and great-grandfather were born in January… you were born in July, dear.”

  11. I’m not entirely sure how you can have a square moat and yet have it be 10 metres wide. It is either square, in which case width at the corners would have to vary, or it is 10m wide in which case it would have rounded corners and therefore not be square …

  12. couple of seconds… being aware of the basic princpile needed… (If you don’t know that and have to figure it out, it may take a while.)

  13. Solved as I was reading.
    Few years ago I went on an adventure holiday, they gave us two planks that were shorter than the gap and told us to get across without touching or falling in the river.
    So that’s where my solution comes from! 😀

  14. I saw a simple 2 plank solution that seemed logical as soon as I scribbled a quick diagram of the setup on a piece of paper. Of course if the raiders were stumped and they’d brought plenty of planks they could always build a giant horse and leave it by the drawbridge…

  15. If the soldiers have run out of urine, why not throw a few thousands of planks into the water instead.

    Oh well, I thought of the 2-plank alternative while I was still reading the problem.

  16. Pole vaulting!!!
    Or swimming, using a soldier as the second half of the bridge, draining the moat, lobbing footbridges at the enemy until they beg for mercy and let the drawbridge down, catching a herring from the moat and using it to fell a nearby tree…

    No idea about the correct, geeky answer though!

  17. have complely no idea. did you solve it by creating something like bridge with two stable points on the ground between the moat (like pontifex?) ?

  18. It dawned on me after a day of not thinking about it!
    It took me about a minute to mathematically confirm it as a viable solution using Pythagoras’ theorem.
    The planks could be a touch shorter than 9.5m and still be viable…
    (In fact, exactly = 20*Sqrt(2)/3 metres; approx 9.428m)

  19. basically,

    we can use T shape for solution.

    A bridge is set horizontally to the land, and the others set up perpendicular to it like a up-down reversed T !

  20. Approach the main gate under the guise of footbridge salesmen. If they ask why they would need footbridges, tell them you’ve surrounded their moat with another moat 9 meters wide.

  21. I used to be afraid to dive into php, but gave it a shot after reading this. Never ever truly adore WordPress’ comment system. Thanks for that tips to obtain me begun.

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