The alliterative poetics used by C. S. Lewis have often proved a critical challenge for scholars without the right kind of medievalist training. In Lewis’s most ambitious contribution to the Modern Alliterative Revival, The Nameless Isle, I argue that he has a strong interest in maintaining fidelity to the Old English alliterative meter, particularly in regards to Sievers types and hypermetric verses, yet he partakes of several adaptations and concessions to Modern English that makes his narrative romance, in some ways, a more successful model than Tolkien’s contemporary “arch-purist” text, The Fall of Arthur. By and large, while questions about a poet’s faithfulness to a historical alliterative tradition are important, deviations from strict fidelity can be even more important yet—they reveal the specific metrical challenges faced by poets in trying to resurrect the alliterative meter. Only by analyzing the metrical data behind a poem like The Nameless Isle can we understand what a revivalist may be specifically trying to accomplish with their revivalism.