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Notes in italics from Hampshire and the Isle of Wight by Nikolaus Pevsner and David Lloyd (1967)
Yale University Press, New Haven and London.

Highclere Castle is the largest mansion in Hampshire. It lies in a perfect park, the work of Capability Brown during 1774-7 ( for Henry Herbert who became Earl of Carnarvon in 1793). . ... At that time the house in the park was a big square classical mansion built for Henry Herbert's uncle Robert Herbert  ... Of this nothing is visible any longer, though Robert Herbert's time survives in the garden temples. The house was remodelled and all but rebuilt by Sir Charles Barry for the third earl in 1839-42. It is in the Elizabethan style, a style in this elaboration still unexpected about 1840. ...  (Barry also designed the Houses of Parliament of about the same date).  Faced in Bath stone.

The house is ashlar-faced, of three storeys with an additional storey in the accentuated parts. They are e.g. on the entrance, i.e. N, side, apart from the corner turrets, the three-bay centre of the eleven-bay front. The windows are of the mullion-and-transom-cross type, with the transoms higher up than in genuine Elizabethan houses. At the top is a strapwork balustrade. The front is much flatter than an Elizabethan front would be. There is in fact very little decoration - just ornamented pilasters in stressed places. There is also the family motto 'Ung Je Serviray' carved above all the ground floor windows.

The W side is the office side. The S side of thirteen bays is still flatter than the side opposite. ... The fenestration on the whole, especially on the N and S sides, are strictly even - Georgian, that is, rather than Tudor. ...

It is the E front where Barry went more opulent. Eleven bays arranged in an ingenious rhythm. Here, apart from the angle turrets, there are yet smaller turrets framing the middle five bays, themselves set in a one-three-one pattern. These smaller turrets are windowless and have arched niches instead. Moreover the bays between them and the angle turrets, while they are boldly recessed from the latter, are flush with the former. ... 

Jackdaws Castle. To the E of the house is the Temple, a strange structure, re-erected before 1743 somehow from a site belonging to Devonshire House in Piccadilly ... Only the Corinthian columns are said to come from Piccadilly. There are six in the front and six at the back of the odd oblong structure, which ends in the short sides internally in two apses. ...

The Monk's Garden and the Secret Garden.
At the south west corner of the castle are the remains of the church of 1689 - outline of walls and bases of pillars.