This article analyzes and interprets an unpublished Spanish architectural treatise that was written for Philip II while he was Prince Regent (c. 1550). The manuscript did not previously attract attention because it was mistakenly described as written for the king's son Philip (later Philip III) in the 1590s. The treatise, which is by no means a masterpiece of architectural literature, derives mainly from L. B. Alberti's De re aedificatoria, but the anonymous Spanish author used his sources selectively to make an explicit connection between morality and a restrained and orderly classical style. He was not concerned so much with buildings as such but rather with their moral dimension and their relevance to the contemporary Spanish state. His concern to articulate the principles of a Catholic, as opposed to pagan, classical style is matched by his eagerness to reform Spanish architectural practice along the lines suggested by Alberti. Simply as an expression of Counter-Reformation aesthetics the treatise is precocious and exceptionally explicit. The treatise was prepared for Philip who, the author states, requested it, which makes it the only known piece of architectural writing prepared especially for him. It brings crucial evidence about Philip's concerns with architecture in the 1550s, before the beginning of the Escorial in 1563, a subject for which historians were reduced to conjecture. The treatise adumbrates a program for reform that is virtually identical to the one Philip actually adopted in 1559. The evidence of the treatise suggests that the strong ideological implications of the style of the Escorial are not simply a post facto gloss but were a deliberate factor in the inception of the design.