Many of them are stuck there because - they spritis do not know they have died - there are not enough guides to show them to the other side - the portals to the other close and trap them between dimensions. The souls might even continue to fight battles in the astral plane (fourth dimension) even though the war ended a long, long time ago.

They may not realize that they have died and will continue to wage battle with other trapped souls. This could result in some degree of unexplained manisfestation on the physical Earth plane - such as bushes moving - unexplained noises , etc.

'Psychic sounds' are by far the most reported phenomena in all cases of haunted areas. Another way of looking at this are - parallel realities - where as in another reality that is parallel to us - the battle is still playing out.

People passing within the frequencies of the area where the battle occurred - will pick up the energies of the battle and the cries of the trapped souls. This will occur with stronger impact on the anniversary of the battle - when a sort of portal opens to those events.

There are psychics / mediums who know how to release the trapped souls - so they no longer haunt the area.