Mines dug in the neighborhood of a castle are also an excellent protection; the deeper and narrower they are, the better it is; and where men are shoving mounted engines toward the walls, it were well if there were many mines. All mines should have a number of small openings, which must be covered so as not to be visible on the surface. They should be filled with fuel of the most inflammable sort, peat or anything else that burns readily. When a castle is attacked at night either from wooden towers or with scaling ladders or any other engine on wheels, the defenders should steal out and fire the mines.[284] Footnote 284: On the subject of mines see Oman, _Art of War_, 549-550. Now if it should happen that the enemy’s stones come over the battlements with such violence that the men cannot remain in the open to defend the wall, it is a good plan to set up strong posts cut from thick oak and to lay large and tough cross beams upon these, then to roof the whole over with firm oak timbers, and finally to cover the roofing with a layer of earth not less than three or four ells in depth, upon which the rocks may be allowed to drop.[285] In like manner the attack of a wooden tower that is moving toward a castle may be foiled by setting up strong, firm posts rising considerably higher than the attacking tower. But a more effective contrivance than all the engines that I have now described is a stooping shield-giant which breathes forth flame and fire.[286] And now we shall close our account of the engines that are useful in defending castle walls with the reminder that every sort of weapon with which one can shoot, hurl, hew, or thrust, and every kind that can be used in attack or defense may be brought into service. Footnote 285: The posts were apparently placed on top of the wall, the purpose being to raise the wall to a greater height as well as to furnish shelter for the defenders. Footnote 286: The shield-giant was probably a mythical device; but it is possible as has been suggested that its fiery breath may refer to the use of Greek fire, with which the Norwegians became acquainted during the crusades, or even to early experiments with gunpowder. Falk, _Altnordische Waffenkunde_, 200-201. It is not known when gunpowder was invented, but the earliest known formula for making it is found in the writings of Roger Bacon, who was a contemporary of the author of the _King’s Mirror_. ------------------------------------------------------------------------ XL THE PROPER MANNERS AND CUSTOMS OF A ROYAL COURT