But there was nothing about their appearance to suggest the heads of a hydra.

They looked like a row of lofty slabs with their upper ends tapered to the shape of a carving-knife point; in fact, the early voyager, ignorant of their great height, might have mistaken them for a rusty old rank of piles that had sagged this way and that out of the perpendicular.
The Peninsula is lofty, rocky, and densely clothed with scrub, or brush, or both.

It is joined to the main by a low neck.

At this junction was formerly a convict station called Port Arthur--a place hard to escape from.

Behind it was the wilderness of scrub, in which a fugitive would soon starve; in front was the narrow neck, with a cordon of chained dogs across it, and a line of lanterns, and a fence of living guards, armed.
We saw the place as we swept by--that is, we had a glimpse of what we were told was the entrance to Port Arthur.