A strict watch is being maintained outside the Elsinore castle because the security of the castle is worried about Fortinbras trying to attack the castle.
Explanation:
The Elsinore castle and Prince Hamlet living inside the castle are being tried to be secured from a possible attack from Prince Fortinbras of Norway.
Prince Fortinbras intends to attack the Elsinore castle to kill Prince Hamlet as a revenge for the killing of his father by Prince Hamlet.
