legal rights wrt erecting secure fence or wall

I have an old unused garden which borders with a public park, the border is simply some overgrown
hedge with lots of holes in it. I now plan to erect a new wall or fence in place of the hedge.
If I have a palisade fence erected are there legal issues, i.e can I be sued if someone
gets injured trying to climb it. Same applies if I erect a barbed wire fence and someone gets caught up in it ?
Even if I simply build a block wall can I be sued if someone falls from the wall ?
The garden has been idle for some years so it has become a hangout for some undesirables

you can build a wall on your boundary at the back or side of your property up to 2m high without planning permission ,i think you would have to plaster or dash the wall to match the area . it poses more problems of graffiti as i found out ,a real pain in the ass.theres no easy solution im afraid especially if the living dead are hanging arround.i put industrial greese and oil mixed on the walls so it keeps them off,they wouldnt dare get their nikes or track suits dirty ,

you can build a wall exempt from PP as long as you follow the
following, (domestic not commercial), be careful on the boundary
location though.

1. The height of any such structure shall not exceed 2 metres or,
in the case of a wall or fence within or bounding any garden or
other space in front of a house, 1.2 metres.

2. Every wall other than a dry or natural stone wall bounding any
garden or other space shall be capped and the face of any wall
of concrete or concrete block (other than blocks with decorative
finish) which will be visible from any road, path or public area,
including public open space, shall be rendered or plastered.

3. No such structure shall be a metal palisade or other security
fence.