John Cowan badly fractured his right ankle after falling into a ha-ha ditch at Hopetoun House on the outskirts of Edinburgh on September 5, 2008, as he and his grandson returned to a car park in the dark.
Mr Cowan, a former water quality manager who was 61 at the time, raised an action seeking compensation at the Court of Session in Edinburgh against the Hopetoun House Preservation Trust and trustees, including the Marquess of Linlithgow.
Damages were agreed at £35,000, but liability was contested and a hearing of evidence was subsequently heard.
A judge has now ruled the trust was liable for the injury suffered by Mr Cowan of Livingston, West Lothian, but the amount should be reduced by 75%. Mr Cowan's five-year-old grandson, Ross, also fell, biting his lip in the incident.
Mr Cowan, who was recovering from an operation for thyroid cancer, had gone on the organised walk to search for bats armed with a torch as instructed. He had never been to the historic house, nor was he familiar with the layout of the grounds.
"I didn't know what a ha-ha was beforehand. It was a [ditch] to keep animals from leaving an area in country parks," he said.
The walk ended at a ranger's centre and Mr Cowan said he immediately took his grandson to the toilet. He did not hear any instructions how to get back to the car park.
Countryside ranger Peter Stevens told the court he had gathered the group inside the centre and given directions to the car park. He did not notice Mr Cowan and his grandson going to the toilet. Had he done so, he said, he would have given individual instruction on returning to the car park.
Lord Bracadale said in his judgment: "I find the pursuer [Mr Cowan] did not hear these directions, no doubt because he was engaged in attending to the needs of Ross. I consider that Mr Stevens must be mistaken in thinking he had checked everyone was present when he gave the directions."
Mr Cowan ended up walking in the dark on a route that took him to a point in the ha-ha where there was a 5ft drop.
Lord Bracadale said he considered the ha-ha was outside the scope of the law over obvious dangers such as cliffs or canals where an occupier was not required to take precautions against a person being injured.
"It is an unusual feature of a concealed nature, particularly in the dark. While the ha-ha was a permanent feature of the landscape at Hopetoun House, it is an unusual feature about which someone crossing the lawn in the dark would be likely to be unaware," he said.
He added: "It seems to me it was reasonably foreseeable a member of the group might, albeit carelessly, take a short cut across the grass to the car park."
The judge said it would have involved no sacrifice to take the group to the front of the house and clearly point out the route.
But he said: "Leaving the path in the dark to walk across open ground was a careless thing to do.
"The pursuer could not have been keeping a proper lookout; if he had, he would have been likely to have spotted the ha-ha when he came across it.
"The top of the ha-ha was edged with stone."