Abattis

ABATIS, in a military sense, is formed by cutting down many entire trees, the branches of which are turned towards the enemy, and as much as possible entangled one into another. They are made either before redoubts, or other works, to render the attacks difficult, ... [An Universal Military Dictionary, Capt. George Smith. London, 1779.]

Examples of how effective these abattis could be are to be found in the military memoirs of 17th, 18th and 19th century soldiers. For example, William Truman, a Confederate artilleryman in the American Civil War, had witnessed an infantry attack that failed due to the impenetrability of an abbatis:

Nov 30th. 1864
About daylight we received the news, that the enemy had left the town and were on their way to Nashville. I told Lt Murphy, that I was going forward to look over the battle field on our front, and he made no objection. I walked straight to the point where our line of battle made the hault, I wanted to settle the question in my mind, why they came back and never returned to the assault again. We battery boys who were looking on and saw the whole performance, had all concluded that there was something else besides the enemy's bullets, that had turned them back, before they reached the breastworks of the enemy, and great was my surprise when I came to the place and found an obstruction, that no body of men, however brave and determined they might be, could pass, and for them to remain there in the face of a muderous fire, by a concealed foe, not more than twenty yards distant, would have been simply suicidal, so they acted wisely by firing a volley and then getting back as soon as possible. But it is reasonable for me to assume, knowing those men to be veterans and thoroughly desciplined, that they were ordered to return, just as soon as their officers saw the situation. Well, the impassable barrier was a brush fence made out of osage hedge, and every man knows that ever saw an osage hedge, that no sensible man would would dare take hold of the brush, with his bare hands as the sharp thorns are very poisenous. This thorny brush fence or abattis was well knit together, and I would call it impassable, and gave our officers and men credit for their wise course, in retreating as quickly as possible. What a pity so many brave men were sacrificed before this matter was discovered. It is a clear demonstration to every officer and private of a complete and inexcusable want of reconnoisance before the attack was ordered. I walked down the brush fence toward the river, hunting for an opening to get on the inside, at last I found one, wide enough for one man to go in and out, made for their pickets to go out and in.
Memoirs of the Civil War, William L. Truman.

Again, in the American Civil War, William Judkins, a Confederate infantry man, was part of a Confederate force advancing into Virginia near Chancellorville in 1863. He recalls his relief in not having to capture the enemy's position, defended as it was by a strong abattis:

Next morning we were ordered around to attack their position, but before we got in position to advance to the attack we found that the enemy had gone; We were glad of that for we never could have taken their position from the side that we were going to attack- for their breast- works were formed in a square on top of a hill, with a great deal of artillery all around, and an abattis all around for 200 yards an all sides- that is the bushes and saplings were cut down with the tops towards the way that we would have had to come, which we never could have gotten through, for the brush and bushes were piled as high as a man's head, and very thick.; the saplings not cut entirely off the stumps. It formed a labyrinth that was impossible to get through, and the enemy could have killed all of us, and we could have done them no harm.
Memoir of William Brock Judkins. The United States Army Military History Institute.