She followed him deeper into the bowels of the cistern – which, devoid of water, seemed more like the underground hall of some dwarf king than a kind of well; and presently she noticed beyond the pool of torchlight another source of light ahead. There was a strong smell, sharp in the nostrils, and under it another, like the trace of rotten eggs. The other light soon grew brighter as they approached and then she heard voices and the light tap-tapping of metal; she saw torches sconced on the walls and pillars, and still further on a murky, green glow.

‘This, Queen Lilla, is why the city will not fall.’

Without warning, there was a sudden rush of heat and light, a roar of flame that made her shrink back, startled. In her shock, she gripped Leo’s arm. The emperor laughed. ‘Don’t worry. You’re quite safe.’

‘Safe? How can anyone be safe around this. . . this—’

‘We call it hydron pyr. Liquid fire.’ The fire-jet had landed with a loud splatter down one of the avenues of pillars, the flames still burning brightly amongst the puddles and damp pavings on the cistern floor. ‘These men manufacture it in quantities sufficient to keep our fleet stocked at all times. Some say it’s the most precious thing the empire possesses. It’s certainly the most deadly.’

‘What’s in it?’

‘Pine resin, naphtha, sulphur. That’s the smell, huh?’ He wafted his hand in the air. ‘These, I know. Many of the other substances required, I do not. I must not. Not even I,’ he exclaimed with a laugh. ‘Since knowing it, I might give that knowledge away. Especially to one so disarming.’ He held up three fingers. ‘Three men only know the exact composition. We call them the “Brilliant Ones”.’ He smiled. ‘Our little joke. Here –’ he beckoned her to follow again – ‘you can meet one of them.’

Lilla followed, feeling both intrigued and excited. There was another great whoosh of flame and this time she wasn’t startled, but watched with silent fascination. She felt a dark thrill at the power of the thing, feeling its heat prick at her face. She thought of her promise to her dead husband. If somehow she could gain the use of this. . . weapon, perhaps then she would be able to keep her word to him.

They approached a far corner of the cistern where the walls were more brightly lit. There were several men working at benches where there were sets of scales and pots of various powders and some larger vats of liquid set on pallets on the ground. In another area, a man was tapping a little hammer at a narrow piece of piping.

‘Nikolaos,’ called Leo softly. A man bent over a set of scales rose from his bench and squinted into the half-gloom. Seeing them, his expression fell. ‘Majesty,’ he gulped and made to kneel.

‘There’s no need for that. Please.’

The man straightened.