At each of the playground's five corners are these adorable plaques designed to attempt to trick kids into learning something. On each plaque are fort-, maritime- and/or Boston-related trivia questions, with clues as to where one might find the answers. Some of these trivia questions are:

When was Castle William erected on Castle Island?
The fort at Castle Island was rebuilt many times after fires.
Castle Island's fifth fort was dedicated in 1702 and named Castle William for England's King William III.

Was there ever a sea monster at Castle Island?
In 1818, sentries reported seeing a sea serpent. There have been other unverified reports (sightings).

What famous American author was stationed at Castle Island?
Edgar Allen Poe was stationed, under an assumed name, with the 1st Artillery at the brick Fort Independence in 1827.