Old Sarum is where Salisbury begins before Salisbury existed: an Iron Age hillfort reused by Romans, Normans and medieval bishops. The earthworks, castle ruins and cathedral foundations sit on a windy chalk rise with wide views over the modern city and surrounding plain.
It is one of the best short trips from Salisbury because the layers are so visible. Walk the ramparts first, then drop into the inner castle and old cathedral footprint; the move from exposed hilltop power to the later cathedral city below becomes obvious.