A few days ago, I walked by a wild growing hedge of blackberries. It was about 10ft wide and higher than me, with long branches interwoven with other blackberries and some other plants growing in it. It would be impossible to walk trough it, even with protective clothing it would be too hard to untangle the branches and form a passage. It was also so dense that I couldn't imagine any animales larger than a cat finding large enough gaps between the branches.
This made me wonder if humans ever used thick hedges, especially thorn hedges, as a wall to defend their settlement. I remember reading in one of Tolkien's books, that one of the larger hobbit towns had an old and dense hedge as wall around it.
If this has been the case, where and when was it done and what plants were commonly used?
Generally in medieval Europe, town defenses were created in a more permanent way, with stone walls or less expensive and easier to construct defenses like wooden palisades, ditches, and earthworks. If fighting in a town or suburb was taking place in the fields and gardens, they might use hedges as improvised walls, but generally a deliberately constructed palisade would be much more effective. However, out in the field, hedges and bushes were seen as a very useful tool for defenders. Soldiers caught in skirmishes might dismount and use hedges as a means of shielding themselves from a direct charge. Armies might cut down hedges and use them to supplement hasty field entrenchments like ditches. These never were intended to completely stop an enemy force in its tracks, but to delay and hamper their maneuvers in order to allow missile troops more time to wear them down and soften them up for the main engagement.
EDIT: Just thought of this, but it might helped to think of the use of hedges as a bit like barbed wire in a modern context: doesn't have anywhere near the protection of concrete barriers or a HESCO barrier, but if it's available, it's much easier to set up.
In many African regions, particularly East Africa this was the primary form of defense for their settlements against animals, called a Boma, it's still used in very rural areas today. The plants (usually acacia branches) aren't usually gown in place but are collected and piled in rings around the settlement.
I've not seen any sources describing it being used specifically against other people, but I'd imagine it could stop a charge, although would be easy to set on fire.
There is a hedge of African Boxthorn planted as part of the defences at Fort Queenscliff, at the entrance to Port Phillip Bay. Fort Queenscliff was built in the 1860s as a fortified artillery point controlling the shipping channels at the approach to Port Phillip Bay as part of defences to the Victorian goldfields.
The Fort sits on top of Shortlands Bluff with the artillery sunk into the top of the cliff pointing out to the bay and the underground magazines between the guns. To protect the guns from land attack, the fort encloses the landward side of the guns with various buildings, a lighthouse and a keep anchoring a curtain wall shaped to provide enfilade fire from firing ports and outside the wall, a dry moat or ditch (now largely filled in).
There was no wall along the cliff top however to the south-east end of the fort where the cliff is shallower, the dry moat continued and is still visible, and the strip between the moat and the top of the cliff is planted with African Boxthorn providing a barb wire type barricade at the top and partly down the slope.