On further consideration of this remarkable group of excessively peculiar features, it becomes evident that they all point more or less obviously to the presence of a double intention in the minds of the constructors of the Brochs. The design of the whole structure and the arrangements of all its separate parts exhibit a careful and laborious adaptation of means and material to the two main objects of shelter and defence. The clever constructive idea of turning the house outside in as it were, placing its rooms within its walls, and turning all their windows towards the interior of the edifice, implies boldness of conception and fertility of resource. The height of the wall, which effectually secured the inmates against projectiles, also removed its essentially weak upper part beyond reach of assault, while the pressure of its mass knit the masonry of the lower part firmly together, and its thickness made it difficult to force an entrance by digging through it—if such a wall could be approached for this purpose when the whole of its upper materials were deadly missiles ready to the hands of the defenders. The door, securely fastened by its great bar, is too strong to be carried by a rush. Placed four feet or more within the passage, it can only be reached by one man at a time, and the narrowness of the passage prevents the use of long levers. In all probability the door itself is a slab of stone, and impervious to fire. But even if it is forced, and entrance gained to the interior court, the enemy finds himself as it were in the bottom of a well 30 to 40 feet in diameter with walls 50 feet high, pierced on all sides by vertical ranges of windows, or loopholes, commanding every foot of the space below, and rising to the number of twenty or more, immediately over the door which gives access to the galleries. In short, the concentration of effort towards the two main objects of space for shelter and complete security was never more strikingly exhibited, and no more admirable adaptation of materials so simple and common as undressed and uncemented stone for this double purpose has ever been discovered or suggested. Perhaps there is no characteristic of the typical structure more remarkable than the extreme constancy of its essential features. The uniformity of plan and construction is so unvarying among all the known examples that there exists no means of tracing the development of the form through a series of primitive or immature stages. In this respect there is a striking analogy between the Brochs and the Round Towers of Ireland. The Irish Towers also appear fully developed, and exhibit a general uniformity of plan and construction which is quite as remarkable in its manifestations among them as it is among the Brochs.[76] Their origin is assignable to peculiar circumstances in the history of the ecclesiastical communities, and chiefly to their constant liability to sudden danger of plunder and murder by roving bands of marauding Norsemen.