Palisades,trenches, and maybe some pit traps with foilage over them?
Those seem like the best medieval era traps at our disposal.
Pits are a good idea, 3man. At the
Battle of Lugdunum, Clodius Albinus used pits to good effect against Severus. Here's an extract from Dio Cassius:
Meantime the soldiers of Albinus arrayed on the
right wing, who had trenches hidden in front of them and pits in the earth
covered over only on the surface, approached as far as these snares and
hurled javelins from a distance. They did not go very far but turned back
as if frightened, with the purpose of drawing their foes into pursuit.
This actually took place. Severus's men, nettled by their brief charge and
despising them for their retreat after so short an advance, rushed upon
them without a thought that the whole intervening space could not be
easily traversed. When they reached the trenches they were involved in a
fearful catastrophe. The men in the front ranks as soon as the surface
covering broke through fell into the excavations and those immediately
behind stumbled over them, slipped, and likewise fell. The rest crowded
back in terror, their retreat being so sudden that they themselves lost
their footing, upset those in the rear, and pushed them into a deep
ravine. Of course there was a terrible slaughter of these soldiers as well
as of those who had fallen into the trenches, horses and men perishing in
one wild mass. In the midst of this tumult the warriors between the ravine
and the trenches were annihilated by showers of stones and arrows.Many would dispute it, but at Lugdunum, three-quarters of the Roman Legions in the entire Empire slugged brutally at each other for two days, and it is possible to assert that the decline of Rome began with the institutional military knowledge and traditions lost when such a large number of veteran legionaries slaughtered each other. That is beyond the point here, though. I'm just agreeing that pits can be very deadly.
Palisades, on the other hand, might make our location seem too foreboding and well-prepared. They control where the enemy can attack, and we want the enemy to feel that we are open and vulnerable on the broad front that faces them. Our flanks should be put against natural obstacles naturally, but maybe our front should be rather inviting of overconfident attack. Maybe not full-blown palisades, just rows of wooden spikes driven into the ground near our lines to break up their formations as they get within halberd's reach. Overall, I like the hidden trenches better.
Have any of our trappers that are with us give tips and advice to the Duke's men, that's the best way to get them thinking of traps.
+1 to this, the experts can think of things we can't, and I believe also we have some siege engineers taught by the late Gunther Agnacious, too. RIP. We should also bring up those loads of the oil we've been planning.