I work as an archaeologist and spend most of the time in the field; I'm currently working in a farmers field which has boundary ditches that are full of brambles. (which are a pain to avoid when crossing through/over)
This question popped into my head as I doubt defensive ditches etc were continually stripped of plantlife and I can imagine that brambles could be somewhat of a deterrent in the form of a natural (albeit weaker) barbed wire.
Speaking only for Southeast Asia, you are absolutely right. As a very cost effective defence, thorny bush featured regularly in Southeast Asian fortifications in two main ways.
Firstly, it was used to fill deep ditches or moats that had dried up. A good example is during the Dutch invasion of Bali of 1841, when thorny bush was used by both sides to fill in their dried up moats. Thorny bush was readily available, and could be gathered and dumped wherever needed to quickly bolster defences.
Secondly, hedges could be planted and used as ‘bio-barbed wire’. In 1877, for example, a fort was constructed out of tree trunks in Laos. Its defences included an open area outside the walls containing bamboo traps, and then around the open area was planted a thick thorny bush hedge. Thorny bush was relatively fast growing and hardy, and needed minimal manpower to maintain. Indeed, the wilder it grew, the better.
In both cases, its purpose was fairly intuitive - it was meant to slow attackers down so they presented an easier target for defending missile troops.
The attacking force had a few options to deal with thorny bush obstacles.
If the attacking force could get close enough, they could cut the bush down or burn it. If it was in a moat, the moat could be filled in with earth.
During Bayinnaung’s siege of Ayutthaya in 1564, for example, Bayinnaung built breastworks, stockades and trenchworks to get close to the city, then used them as cover to transport enough earth to build causeways across the moat.
Alternatively, an attack could be launched at night or during thunderstorms in the day. If they attacked quietly enough, they might be able to get past a couple of obstacles before being discovered. If not, the defenders would have difficulty aiming. If it was raining, working with gunpowder weapons would be more difficult.
If an attacking force opted to storm the fortifications, thorny bush hedges could be overcome through the use of large shields laid over them, creating pathways for troops to pour through. This is similar to how medics’ stretchers can be laid over barbed wire in modern warfare. Moats of thorny bush could be crossed through use of bamboo causeways that could be prepared in advance and then thrown over the thorns.
Charney, M. (2004). Southeast Asian Warfare, 1300-1900. Leiden, The Netherlands: Brill.