Peveril Castle (Derbyshire)
In the central northern area of the Peak District, this impressive (but gauntly ruined) castle was started by William Peveril soon after the Norman Conquest, important even with its bleak surroundings because of the extensive lead mines in that district, which of course went along with the feudal grant. Bolsover Castle to the east was also a 'Peveril' castle, of a similar pattern, but the contrast couldn't be more different -- one ended up a terrible ruin, the other a grand 17th Century showcase (that's ruined too, but not in the same way). This agriculturally poor area was always important, even in the Bronze Age -- hence the huge hill fort of Mam Tor nearby, now a great site for hang-gliding -- and in Roman times, when the rare Blue John semi-precious stone was discovered.
This is, for all its totally ruined state, one of the most impressive 'mountain' castles in England. And it takes a major effort just to huff-and-puff your way up the hill to inspect the ruins. Two notable points: No major improvements were made in the later Middle Ages to bring the fortress up to date, so what remains is basically Norman; there is a very large cave underneath the keep entered from the chasm on the other side of it (not shown pictured here). The castle is built on a steep ridge with a deep slope on the town side, as seen here, and a sheer drop on the other.
As a structure, this is a very simple Norman castle -- not a motte and bailey as was most common in the early years of the conquest, but basically just a strong keep tower and a stone-walled bailey. Most of the interior buildings would have been timber-framed, and have long disappeared. Given the slope of the site, it could never have become a comfortable manorial center, so was probably always dedicated to some sort of oppressive military purpose over the local populace. I suppose Walter Scott's Peveril of the Peak is partly based here -- but I have never steeled myself up to read that book.
The picture below (gray-scaled, not reduced to two colors) gives a more mellow picture of a typical ruin. Given the touristy aspect of Castleton in the High Peak, that is appropriate, but this place could never earlier been anything but grim.