Article 1, Section 8, Clause 17
William Blackstone, Commentaries 1:2551765
This statute, it is obvious to observe, extends not only to fleets and armies, but also to forts, and other places of strength, within the realm; the sole prerogative as well of erecting, as manning and governing of which, belongs to the king in his capacity of general of the kingdom: and all lands were formerly subject to a tax, for building of castles wherever the king thought proper. This was one of the three things, from contributing to the performance of which no lands were exempted; and therefore called by our Saxon ancestors the trinoda necessitas: sc. pontis reparatio, arcis constructio, et expeditio contra hostem. And this they were called upon to do so often, that, as sir Edward Coke from M. Paris assures us, there were in the time of Henry II 1115 castles subsisting in England. The inconvenience of which, when granted out to private subjects, the lordly barons of those times, was severely felt by the whole kingdom; for, as William of Newbury remarks in the reign of king Stephen, "erant in Anglia quodammodo tot reges vel potius tyranni, quot domini castellorum:" but it was felt by none more sensibly than by two succeeding princes, king John and king Henry III. And therefore, the greatest part of them being demolished in the barons' wars, the kings of after times have been very cautious of suffering them to be rebuilt in a fortified manner: and sir Edward Coke lays it down, that no subject can build a castle, or house of strength imbatteled, or other fortress defensible, without the licence of the king; for the danger which might ensue, if every man at his pleasure might do it.
The Founders' Constitution
Volume 3, Article 1, Section 8, Clause 17, Document 1
The University of Chicago Press
Blackstone, William. Commentaries on the Laws of England: A Facsimile of the First Edition of 1765--1769. Chicago: University of Chicago Press, 1979.
Easy to print version.