Question: What structure that sits on a cliff overlooking the River Ness appears on the reverse of The Royal Bank of Scotland £50 note, is a banknote of the pound sterling he second largest denomination of banknote issued by The Royal Bank of Scotland that also features Lord Ilay?

Context paragraphs:
The Royal Bank of Scotland £1 note: The Royal Bank of Scotland £1 note is a banknote of the pound sterling.  It is the smallest denomination of banknote issued by The Royal Bank of Scotland.  The current cotton note, first issued in 1987 bears an image of Lord Ilay, one of the founders of the bank, on the obverse and a vignette of Edinburgh Castle on the reverse.

Bank of Ireland £50 note: The Bank of Ireland £50 note is a banknote of the pound sterling.  It is the second largest denomination of banknote issued by the Bank of Ireland.

The Royal Bank of Scotland £50 note: The Royal Bank of Scotland £50 note is a banknote of the pound sterling.  It is the second largest denomination of banknote issued by The Royal Bank of Scotland.  The current cotton note, first issued in 2005 bears an image of Lord Ilay, one of the founders of the bank, on the obverse and a vignette of Inverness Castle on the reverse.

The Royal Bank of Scotland £20 note: The Royal Bank of Scotland £20 note is a banknote of the pound sterling.  It is the third largest denomination of banknote issued by The Royal Bank of Scotland.  The current cotton note, first issued in 1987 bears an image of Lord Ilay, one of the founders of the bank, on the obverse and a vignette of Brodick Castle on the reverse.

Inverness Castle: Inverness Castle (Scottish Gaelic: Caisteal Inbhir Nis) sits on a cliff overlooking the River Ness, in Inverness, Scotland.  The red sandstone structure evident today was built in 1836 by architect William Burn.  It is built on the site of an 11th-century defensive structure.  Today, it houses Inverness Sheriff Court.  There has been a castle at this site for many centuries.  The castle itself is not currently open to the public, however a recent campaign has led to the creation of a working group to explore the possibility of doing so in the future.  In April 2017 the north tower of the castle was opened to the public as a view point.  At present, only the castle grounds and the north tower are open to the public.

The Royal Bank of Scotland £5 note: The Royal Bank of Scotland £5 note, also known as a fiver, is a banknote of the pound sterling.  It is the second smallest denomination of banknote issued by The Royal Bank of Scotland.  The current polymer note, first issued in 2016, bears an image of the author Nan Shepherd on the obverse and a pair of mackerel on the reverse.

Clydesdale Bank £50 note: The Clydesdale Bank £50 note is a banknote of the pound sterling.  It is the second largest denomination of banknote issued by the Clydesdale Bank.  The current cotton note, first issued in 2009 bears a portrait of Elsie Inglis, founder of the Scottish Women's Hospitals, on the obverse and an image of the Antonine Wall on the reverse.

The Royal Bank of Scotland £10 note: The Royal Bank of Scotland £10 note, also known as a tenner, is a banknote of the pound sterling.  It is the third smallest denomination of banknote issued by The Royal Bank of Scotland.  The current cotton note, first issued in 1987 bears an image of Lord Ilay, one of the founders of the bank, on the obverse and a vignette of Glamis Castle on the reverse.  A new polymer note featuring a portrait of scientist Mary Somerville on the front and a pair of otters on the reverse will begin circulating later in 2017.

Bank of Scotland £50 note: The Bank of Scotland £50 note is a banknote of the pound sterling.  It is the second largest of five banknote denominations issued by the Bank of Scotland.  The current cotton note, first issued in 2007 bears the image of Walter Scott on the obverse and a vignette of the Falkirk Wheel on the reverse.

The Royal Bank of Scotland £100 note: The Royal Bank of Scotland £100 note is a banknote of the pound sterling.  It is the largest denomination of banknote issued by The Royal Bank of Scotland.  The current cotton note, first issued in 1987 bears an image of Lord Ilay, one of the founders of the bank, on the obverse and a vignette of Balmoral Castle on the reverse.

Supporting paragraphs: The Royal Bank of Scotland £50 note Inverness Castle
Answer: Inverness Castle