The challenge is aimed at young people aged 11 to 18, who are encouraged to work in teams and use “lateral thinking, ingenuity and perseverance” to crack the seven brainteasers set by GCHQ’s “in-house puzzlers.

The challenges require codebreaking, math, lateral thinking, and a bit of ingenuity to reveal the name of a famous UK landmark. There are also three additional hidden elements in the pictures for those seeking an extra challenge.

What does a spy agency give for Christmas? How about a riddle wrapped in an enigma inside a mystery. GCHQ, Britain’s electronic and cyber-intelligence agency, on Wednesday published its annual Christmas Challenge – a seasonal greeting card that doubles as a set of fiendishly difficult puzzles designed to excite young minds about solving cyphers and unearthing clues.

This year, three extra clues, known as Easter eggs, will be hidden among the seven challenges, each of which will lead the puzzlers to a UK landmark. GCHQ’s chief puzzler, Colin, has also hidden a secret message in the following quote.