To begin with, the Mezentines were, as one of them put it later, like a widow killing a chicken. They slashed wildly at the sappers' heads and arms, frantically trying to get the loathsome job over and done with, desperate not to touch or come into contact with the scrambling, wriggling bodies of their enemies. As a result, they killed few of the sappers but wounded all of them; long, slicing cuts to the scalp that sprayed blood like a cow pissing, chunks hacked out of shins and elbows-the pain revolted them, and made them flail even more furiously, their arms held out straight to maximise the distance (which meant more cuts were made with the tip than the edge; the wound the fencing books called a stramazone, designed to hurt and infuriate an enemy into making an error). Some Mezentines hit hard enough to sever hands or feet, others struck at too shallow an angle, so that the blade skipped off the scalp and sliced off an ear or a nose; results so grotesque that the Mezentines were sick with horror at what they'd done, and lashed out even more to put an end to the nightmare as quickly as possible.
All of which took time; rather longer than planned. Also, the relief shift (not being soldiers) failed to rush to their comrades' assistance, as they were supposed to do. Instead, as soon as they realised what was happening, they scampered back to the camp and called out the guard. The Mezentines waiting in ambush didn't know that, of course, and the ambush itself went flawlessly. Their attack (in rear, in the cramped trench) went home exactly as planned, but the Eremian and Aram Chantat infantry who'd answered the relief sappers' call were adequately armoured, though not as well as the Mezentines, and knew in a matter of seconds what was going on. They turned and fought back. The Mezentines, suddenly finding themselves facing line infantry instead of men in thin shirts with shovels, immediately tried to back away, but the sides of the trench were too steep for heavily armoured men to climb. After half a dozen had been cut down without making any effort to fight back, they rallied and launched a wild, completely unscientific counter attack. They did relatively little damage with their weapons, but their armour was too strong to be easily penetrated or smashed open, particularly in a crowded, narrow trench with no room for a really good swing. Accordingly, the allies jabbed and bashed at them but couldn't actually kill them, and they flailed and walloped back to more or less the same effect. Only when they were too tired to keep up their windmill assaults could the allies get close enough to find the weak joints and hinges in their armour; and by then, the Mezentines behind the fighting line had had the wit to pile up a few empty gabions into a makeshift stair, to get them out of the trench.
Because the guard was busy elsewhere, the two platoons sent to die nobly for their country got to the mouth of the trench and found nobody there to meet them. They stood about for a while, horribly afraid they'd messed up and gone to the wrong place. Then their nerve broke, and they ran back the way they'd just come, meeting up with the first gush of fugitives from the ambush party; who in turn assumed they must be reinforcements, and (bravely and with agonising reluctance) tried to climb back into the trench and continue the fighting. The result would have been very bad for them if a handful of men from the two platoons hadn't looked down on the helmeted heads milling around in the trench below them, and promptly started pelting them with rocks. Fortuitously (it could have gone either way), the heads they battered were Eremian and Aram Chantat rather than Mezentine, and the shock of the unexpected hail of missiles from a quarter they'd not expected trouble from prompted the allies to pull back out of the fighting, letting the surviving Mezentines in the trench get out safely.
None of which mattered, of course. The real objective of Psellus' plan was to let an assault party loose undisturbed in the new trench, which he correctly guessed was being built to take heavy machinery up to the ditch. Well aware that their time was limited, they set to with the ferocious determination of very frightened men. They smashed gabions, cut open fascines, scattered the brushwood in big heaps and set fire to them; they grabbed shovels or pulled off their helmets to scoop dirt with, and remarkably quickly managed to undermine the supports of the earth bank, so the spoil slid back into the trench and filled it. They even rolled back into the trench a huge boulder, which had taken two shifts of sappers a whole day to prise up and haul out of the way. As they were doing this, it occurred to them that they'd been rather longer than they'd anticipated. The two platoons should've been shredded and swept aside by now, and enemy soldiers should be pouring into the parallel, their cue to stop work and run. But no soldiers came, and they started to worry. Did the absence of the enemy mean that the plan had been wholly subverted; was there an enemy force waiting to intercept them on their way back, rather than engage them in the trench? By this point they'd done as much damage as they could without additional equipment. They had no idea what was happening, only a vague feeling that something had gone wrong and they were in a different danger than the one they'd been expecting to face. After a brief, slightly hysterical discussion, they decided that they couldn't trust their planned escape route any more, which meant they had no choice but to go back the way they'd come, down and along the approach trench…
By the time they ran into the enemy, the bombardment with rocks from outside the trench had stopped, and the allies were trying to make up their minds whether to press on and try and rescue the sappers, as they'd originally intended to do before the ambush hit them, or call it a day and go back. The sudden appearance of yet more Mezentines, coming up the trench, was rather more than they could cope with. The Eremian lieutenant who was now the ranking officer (the Aram Chantat captain had been one of the few allied casualties who'd actually died on the point of a Mezentine sword) ordered his men to pull back, intending to form a shield-wall at the point of the zigzag, where there was rather more room to deploy. But the surviving Aram Chantat officer took this for cowardice in the face of the enemy and ordered his own men to push the Eremians out of the way and attack the Mezentines at once. What happened next was never quite clear. The Eremian lieutenant maintained that a party of the Mezentines outside the trench, the ones who'd thrown the rocks, crept up along the bank while the two allied contingents were scuffling and contrived to undermine the gabions and loose spoil, bringing the bank down on the allies' heads. Other survivors maintained that in the course of the scuffle, the gabions at the base of the bank were dislodged, and that was what caused the bank to collapse. In any event, a good third of the allies were under the landslip, and when the dust settled, both sides found themselves separated from the enemy by a solid wall of earth.