What does "come up on" mean here?

He gave her shoulder a reassuring pat. “You best mount up, milady. It’s a long day’s ride to Acorn Hall, but at the end of it we’ll have a roof above our heads and a hot supper in our bellies.”
It was a long day’s ride, but as dusk was settling they forded a brook and came up on Acorn Hall, with its stone curtain walls and great oaken keep. (Game of Thrones)

Context: the travelers were expecting to go to Acorn Hall - they planned to go there.
Hi. Does "come up on" equal "come upon" here, which means "find something unexpectedly"? If so, I find it a bit illogical. So what does "come up on" mean here?

Thank you.