‘FOR IF ANYONE IS CAST into it, the law permits no mention of him to be made thereafter, but death is the penalty for the man who speaks his name.’ Many years ago, when I first read Procopius (1.5.7–6.10), I knew the castle of silence had to be the basis of a novel.

The only specific modern study of which I am aware is G. Traina, ‘La forteresse de l’oubli’, Muséon 115 (2002), 399–422.

The historical fortress-prison was not in the Elburz Mountains. The events of this novel suggest why the King of Kings was wise not to locate it near a frontier, although in Procopius a fugitive manages to make his escape to the Steppe.

Sassanid Persia

Good places to begin to learn about Sassanid Persia are: J. WiesehÖfer, Ancient Persia from 550 BC to 650 AD (London and New York, 1996); M. Brosius, The Persians: An Introduction (London and New York, 2006); and T. Daryaee, Sasanian Persia: The Rise and Fall of an Empire (London and New York, 2009).

The translated documents and commentary in B. Dignas and E. Winter, Rome and Persia in Late Antiquity (Eng. tr., Cambridge, 2007), offers a stimulating and different approach.

On the Sassanid army, see: M. Whitby, ‘The Persian King at War’, in E. Dabrowa (ed.), The Roman and Byzantine Army in the East (Cracow, 1994), 227–63; and two articles in B. Campbell and L. A. Trittle (eds.), The Oxford Handbook of Warfare in the Classical World (Oxford, 2013) – S. McDonough, ‘Military and Society in Sasanian Iran’ (601–20) and A. D. Lee, ‘Roman Warfare with Sasanian Persia’ (708–25). A splendidly illustrated guide for the non-specialist is D. Nicolle, Sassanian Armies (Stockport, 1996). Some of Nicolle’s attribution of images are corrected by St. J. Simpson in a review in Antiquity 71 (1997), 242–5.

The Frumentarii

The frumentarii were the closest thing the Roman Emperor had to a secret service. Soldiers seconded from other units, with a base on the Caelian Hill in Rome, they were his confidential messengers, spies and assassins. Their obscurity in scholarship is so profound that they lack an entry in the fourth edition of The Oxford Classical Dictionary (2012), but since the publication of my novel Fire in the East (2008), they seem to have become almost ubiquitous in Roman historical fiction. A cautious introduction to the historical frumentarii is C. J. Fuhrmann, Policing the Roman Empire (Oxford, 2012), 152–5.

Mesopotamia and the Roman East

F. Millar, The Roman Near East 31 BC–AD 337 (Cambridge, Mass. and London, 1993) remains the best one-volume study.

The Walls of Alexander

The dating of these defensive structures, the longest of which stretches for over 120 miles, has long been controversial. For the purposes of this story I have blithely ignored the recent archaeological study that dates them to the fifth century AD: E. Sauer et al., ‘Linear Barriers of Northern Iran: The Great Wall of Gorgan and the Wall of Tammishe’, Journal of the British Institute of Persian Studies 44 (2006), 121–73; available online at https://www.research.ed.ac.uk/portal/files/.../Linear_Barriers_of_Northern_Iran.pdf

Manichaeism