The ground shook as a huge fireball ignited on the roof of the tower. It was so bright that it shadowed the receding pool of flame near the assault force.

Ever since the gallery began its tortoiselike advance, the tribune had been too caught up in his immediate surroundings to think about the larger aspects of his plan. The professionals of the legion, rank and file as well as the centurions, had done their job with the stolid excellence of a grist mill grinding away its allotted task.

When Rusticanus gave the signal, two soldiers poured their jug of enhanced naphtha into the breech of the log. The local officer who advised on the process had suggested igniting it with water to spark on the quicklime. The legionaries had chosen to risk an open flame instead, something they understood as they did not understand starting fires with water.

The centurion in charge stepped to the log when the legionaries jumped back. He thrust a torch into the funnel glistening with the residue of thickened fluid. Fire bloomed from the touchhole.

"Bellows!" ordered Rusticanus.

Horns and trumpets cried out in a cacaphony intended to terrify the enemy rather than communicate orders. As the sky echoed, the twenty strong men on each lever of the bellows began to stride forward, ramming the air in the oxhide chamber into the base of the hollow log and the fire already blazing there.

Flame spurted twenty feet in the air from the touch-hole before a pair of soldiers clamped a bronze plug down on it. The air surging from the bellows mixed with the fluid and rammed it toward the open end of the tube over a hundred feet in the air. When oxygen bubbled into and through the burning liquid, the combination puffed explosively up the hollow trunk-and out, in an orange-red flash, across the defenders on the top of the tower.

The spurt was of superheated gas, not fluid that clung with the tackiness of pitch and molten sulphur, but it crinkled the bowstaves, armor, and faces of those archers it enfolded as they crouched at embrasures. The two defenders pouring liquid through a spout wailed and dropped their open vat as flame burst from it to meet the puff expanding from the hollow log.

There were several hundred additional gallons of fluid on the tower in closed containers which shattered when spreading fire wrapped them. When half the jars had ignited in a matter of seconds, the remainder exploded simultaneously. Part of the crenellations, fragments of equipment, and the bodies of defenders too fiercely ablaze to be recognized as living things rained in all directions from the top of the tower.

The flare mounted in a hemisphere, like the cap of a mushroom thrusting itself through the loam, until it broke free of the stone and wrapped in upon itself to climb still higher on the reflected heat of its own cumbustion. The platform from which it had lifted was bare of any form of life save the few defenders who still thrashed in the blazing sheet which had devoured their eyes and lungs already.

Vibulenus knew his weapon had been successful when an object slammed the sloping roof of the gallery and bounced, then fell again to the ground before the assault force. It could have been a burning missile, heavy enough that its shock grounded the shelter again. What sprawled in a smokey wrapper of flames was not a timber, however, but a corpse that had been a cross-bowman before his flesh melted and heat cracked the phial in which he dipped his quarrels. The resinous poison burned blue.

"All right!" ordered Clodius Afer in a voice burned skeletal by emtion and flame-dried air. "Left on the count, boys, and put your backs-"

"Out the front with your tools, men," said Vibulenus, speaking from a mind where everything had a place, like the markers of a board game awaiting the next shake of the dice cup.

It did not'occur to him that he was countermanding the centurion. He was placing his game pieces in the illuminated security of his imagination. The dark and bloody reality-of which his body was a part-did not impinge on what was right from a standpoint of command.

"We're safe close by the wall if we move fast," the tribune shouted. When his words had no effect for a further long moment save to turn heads toward him, he added, "Move!" and prodded the ribs of Clodius and Niger.

"Come on, soldiers!" roared the centurion, ducking under the crossbar with the jerky certainty of a boulder rolling downhill-after the tribune pushed it. "Let's take this fucker down!'

No one else in the mobile gallery could get out the front until the leading row clambered free. Those men wouldn't have been in the front rank unless they were willing to leave cover. They scrambled from under the shelter, and Vibulenus followed them in the irrational certainty that the remainder of the assault force was coming also. He was playing a complex game of Bandits, and they were the carved-stone counters on the board moving as he willed.

For that matter, they did follow him-every man of the assault force, because they were Romans… and they were soldiers… and they were, by all the gods, being led.

The tower was a sullen candle with a pillar of flame above the streakes of blazing fluid crawling through the stonework and arrow-slits of the upper stories. The lowest twenty feet of the wall had been built without openings, and even above that level many of the embrasures had been bricked up against side effects of the defenders' own flame weapons. With the top of the tower a dripping inferno, the ground near the base of the structure was a dead zone which none of the weapons in the fortress could reach.

The outer world swept back over Vibulenus as he squirmed out of the gallery's dark and stinking cover. Heat had sources again instead of being a dull ambiance. The gout that had splashed before the gallery was now shrunken to a handful of sulphurous pools to the right side, and the body of the archer-also shrunken-lay for the tribune to leap as the quickest way to the wall and greater safety.

The hollow treetrunk was a slash against the sky, its muzzle-end rimmed with tiny flames. Vibulenus hoped they would not pour another jar of fluid into its breech in order to repeat the process. At the time he planned the attack, multiple spurts of flame had seemed both necessary and reasonably safe. He had not fully appreciated the way the fire clung like a solid thing wherever the fluid had ignited. The interior of the great tube must contain thousands of hot spots which would turn a fresh draft of fluid into a fireball at the breech end this time.

But that was the concern of others, while the wall was a matter for Vibulenus and the nineteen men with him.

The lower rows of that wall were blocks two feet high and three across. Their thickness was concealed until the first one was prised out, and Clodius Afer was already organizing that. The centurion wedged the thicker edge of his pick-mattock into one vertical crack while Niger and another legionary ran their crescent-bladed turf-cutters over the upper and lower surfaces of the block against which he was prying.

Vibulenus chopped the mattock blade of his own tool against the remaining edge of the block so that he and Clodius could thrust against one another. They all carried ordinary pieces of entrenching equipment, though some of the soldiers began using their swords because the blades reached deeper into the interstices of the wall. The blocks themselves were of fine-grained stone which showed no tendency to split or shatter, but the mortar in which they were laid had burned to powder.

Clodius gave a shout and leaned sideways against the head of his tool, levering that end of the block three inches from the line of the wall in a shower of gritty mortar. The tribune shouted also in unconscious imitation and thrust back, using the greater leverage of the helve. Blood and pus from his blistered palms gleamed on the hickory shaft, but Vibulenus did not notice it. The stone, already loosened and held by decreasing friction as more of it was tugged clear of its fellows, shifted even farther than it had at the centurion's thrust.