Chapter II: Mary Queen of Scotts
Queen Elizabeth I of England (1533-1603) was Protestant and was always wary of Catholic plots to seize her throne. Her cousin Mary Queen of Scots (aka Mary Stuart) was Catholic and was often the focus of plots to depose Elizabeth. So Elizabeth had Mary locked up in a castle -- Elizabeth was hesitant to execute her without good cause.
Eventually Mary and her supporters found a way of communicating by hiding messages in the bung of a beer barrel to escape detection by her guards. She used a simple letter substitution code as shown. Unbeknownst to her, Elizabeth was secretly intercepting and translating all her messages.
Letter substitution codes do not need to use symbols. Letters can just be replaced by other letters. The Caesar Shift code is a very simple letter substitution code. The puzzles you are solving are simple letter substitution codes.