An Adventure in Lisbon 22

The twins' class welcomes a new student who has transferred from another school. His name is Eduardo, he's very handsome and has special talents, such as making castles out of chewed paper.
He also set up a pirate radio station in the attic, from which he broadcasts surprise programmes that include interviews and voice impersonations. When he invites the group to visit the studio, he accidentally tunes into a conversation about a treasure hidden two hundred years ago in a monument in Lisbon. But which one, since there are so many monuments in Lisbon? The answer lies in a difficult-to-decipher enigma that the AL gang also heard.
The treasure hunt leads them to spend a night locked in the Belém Tower and to take a dip in the icy, muddy waters of the Tagus.