Though there was no need to take down and measure the staff, it is undoubtedly necessary to find its height before the answer can be given. 
It was well known among the friends and retainers of Sir Hugh de Fortibus that he was exactly six feet in height. 
It will be seen in the original picture that Sir Hugh's height is just twice the length of his shadow. 
Therefore we all know that the flagstaff will, at the same place and time of day, be also just twice as long as its shadow. 
The shadow of the staff is the same length as Sir Hugh's height; therefore this shadow is six feet long, and the flagstaff must be twelve feet high. 
Now, the snail, by climbing up three feet in the daytime and slipping back two feet by night, really advances one foot in a day of twenty-four hours. 
At the end of nine days it is three feet from the top, so that it reaches its journey's end on the tenth day.

The reader will doubtless here exclaim, "This is all very well; but how were we to know the height of Sir Hugh? It was never stated how tall he was!" 
No, it was not stated in so many words, but it was none the less clearly indicated to the reader who is sharp in these matters. 
In the original illustration to the donjon keep window Sir Hugh is shown standing against a wall, the window in which is stated to be one foot square on the inside. 
Therefore, as his height  :will be found by measurement to be just six times the inside height of the window, he evidently stands just six feet in his boots!