But if the concept was simple, the practicalities had been plagued by complexities. Ceilings collapsed under the weight of armed men, walls gave way, wheels splintered and worst of all, once over thirty feet high the structure was unstable and prone to tip.
Khitan recalled how for more than a year he had worked harder than his slaves, sleeping less than three hours a night. He had strengthened the ceilings, but this had merely made the entire structure more heavy and less stable. In despair he had reported to Ulric. The Nadir warlord had sent him to Ventria, to study at the University of Tertullus. He felt that he had been disgraced, humiliated. Nevertheless he obeyed; he would suffer anything to please Ulric.
But he had been wrong and the year he had spent studying under Rebow, the Ventrian lecturer, proved to be the most glorious time of his life.
He learned of mass centres, parallel vectors and the need for equilibrium between external and internal forces. His appetite for knowledge was voracious and Rebow found himself warming to the ugly Nadir tribesman. Before long the slender Ventrian invited Khitan to share his home, where studies could be carried on long into the night. The Nadir was tireless. Often Rebow would fall asleep in his chair, only to wake several hours later and find the small, one-armed Khitan still studying the exercises he had set him. Rebow was delighted. Rarely had a student showed such aptitude, and never had he found a man with such a capacity for work.
Every force, learnt Khitan, has an equal and opposite reaction, so that, for example, a jib exerting a push at its top end must also exert an equal and opposite push at the foot of its supporting post. This was his introduction to the world of creating stability through understanding the nature of stress.
For him the University of Tertullus was a kind of paradise.
On the day he had left for home the little tribesman wept as he embraced the stricken Ventrian. Rebow had begged him to reconsider; to take a post at the university, but Khitan had not the heart to tell him he was not in the least tempted. He owed his life to one man, and dreamed of nothing but serving him.
At home once more, he set to work. Under construction the towers would be tiered, creating an artificial base five times the size of the structure. While being moved into position, only the first two levels would be manned, creating a mass weight low to the ground. Once positioned by a wall, ropes would be hurled from the centre of the tower and iron pins hammered into the ground, creating stability. The wheels would be iron-spoked and rimmed, and there would be eight to a tower, to distribute the weight.
Using his new knowledge, he designed catapults and ballistae. Ulric was well pleased and Khitan ecstatic.
Now, bringing his mind back to the present, Khitan climbed to the top of the tower, ordering the men to lower the hinged platform at the front. He gazed at the walls three hundred paces distant and saw the black-garbed Deathwalker leaning on the battlements.
The walls were higher than at Gulgothir and Khitan had added a section to each tower. Ordering the platform to be raised once more, he tested the tension in the support ropes and climbed down through the five levels, stopping here and there to check struts or ties.
Tonight his four hundred slaves would go to work beneath the walls, chipping away at the rocky floor of the Pass and placing the giant pulleys every forty paces. The pulleys, six feet high and cast around greased bearings, had taken months to design and years to construct to his satisfaction, finally being completed at the ironworks of Lentria’s capital a thousand miles to the south. They had cost a fortune and even Ulric had blanched when the final figure was estimated. But they had proved their worth over the years.
Thousands of men would pull a tower to within sixty feet of a wall. Thereafter the line would shrink as the gap closed; the three-inch diameter ropes could be curled round the pulleys, passed under the towers and hauled from behind.