I've always been intrigued with ancient/ medieval warfare, the simplicity and brutality of the methods used hold a great appeal to me. Especially with regards to the simple yet ingenious force multipliers utilized by our ancestors.
Dead falls were commonly used, these pits were used to kill wolves and bears, which at this point commonly preyed on people and especially after wars which supplied plenty of carrion, these things were huge. I've read that during the 100 years war wolves in Northern France were recorded near 300 pounds, and bold enough to attack a man on horseback.
A pit filled with sharp stakes, 5-6 foot deep would be nasty surprise. for both horse and foot, and in my opinion would be a good center peice for an ambush.
Another weapon and precursor to the landmine is the Roman Stimuli, this is basically an arrowhead or small knife blade with a barb, the 3-4" tang is bent at right angles and driven into a stake which is then drove into the ground, not only did these things wound they could easily kill an enemy who happened to fall on them.
The caltrop needs no intro, with a hollow tubed version of this weapon in use by traffic police to this day, the hollow design allows for tires to deflate quicker. These can be laid quicker than the stimuli and they always land with a point up.
While not expedient in the feild, its worth a mention that during Alexanders seige of Tyre, the defenders heated sand on the walls and dumped the white hot sand on the Macedonian troops, the sand would find its way under armour, burn through clothing and Herucles help the warrior who looked up when it fell
...Good wall defense that 
Another simple idea I've always admired is the simple earth work with sharpened stakes angled gut high, not as much cover as a palisade but it would effectively keep horse and foot from over running a position and cost them some momentum, allowing them to be speared or shot before they could close en masse.
~RDM~
