You will first need to establish what permissions, if any, you may need e.g. from a landlord, or planning permission from the local authority (because of the appearance or height to which it will raise the wall or fencing it, or the premises are located within a conservation area).
Make sure that any such "toppings" will be clearly visible over their whole length, will all be at well above normal head height and will not overhang any public area, road, pavement or footpath, etc. Warning signs should be considered.
Read more about perimeter and building defences