To get to your destination, enter Christ’s College via the door in the Great Gate on St. Andrew’s Street. Walk around the green and to an arch in the Fellow’s Building at the far right corner.
Go through that into the Fellows’ Garden.
Walk directly across the grass or follow the path either way around to the far end. There, there are two mulberry trees, the older of which was planted in 1608 and both of which still fruit every year.
Here’s a close-up, deep in the mound: