Assange notes in the letter that it was "only fitting to extend a heartfelt invitation" to his "liege" as the monarch prepares to be officially crowned king, months after the death of his predecessor and mother, Queen Elizabeth II.
"I thought it only fitting to extend a heartfelt invitation to you to commemorate this momentous occasion by visiting your very own kingdom within a kingdom: His Majesty’s Prison Belmarsh," the letter reads. "After all, one can truly know the measure of a society by how it treats its prisoners, and your kingdom has surely excelled in that regard."
The letter, which was published on the Declassified UK website, sees Assange point out how the UK is the country with the largest number of prisoners in Western Europe. He also noted the living conditions in the prison are bad, mentioning the poor quality of food and sanitation.
"During your visit, you will have the opportunity to feast upon the culinary delights prepared for your loyal subjects on a generous budget of two pounds per day," the letter continues.
"Savour the blended tuna heads and the ubiquitous reconstituted forms that are purportedly made from chicken. And worry not, for unlike lesser institutions such as Alcatraz or San Quentin, there is no communal dining in a mess hall. At Belmarsh, prisoners dine alone in their cells, ensuring the utmost intimacy with their meal."
The WikiLeaks founder also noted that inside the prison walls, "you will find the most isolated place within its walls: Healthcare, or “Hellcare” as its inhabitants lovingly call it."
"Here, you will marvel at sensible rules designed for everyone’s safety, such as the prohibition of chess, whilst permitting the far less dangerous game of checkers," he continued.
"Deep within Hellcare lies the most gloriously uplifting place in all of Belmarsh, nay, the whole of the United Kingdom: the sublimely named Belmarsh End of Life Suite. Listen closely, and you may hear the prisoners’ cries of 'Brother, I’m going to die in here', a testament to the quality of both life and death within your prison."
Assange concluded the correspondence by underscoring that a visit from Charles "is an honour befitting a king."