There are several great metaphors in Hamlet's speech to Horatio during which Hamlet explains the weaknesses of man ("oft it chances in particular men") and where those weaknesses come from.
In a metaphor that compares the imperfections in man's nature to the tunnels destructively raised up by moles, he says that there may be "some vicious mole of nature" in man that destroys him. Moles are tunneling mammals. Their raised tunnels destroy picturesque lawns from below. The damage occurs without there being a destructive influence in sight. These tunnels present the "o'ergrowth of some complexion"--the unexpected tunnels of some sort--that break down "the pales and forts of reason."
Hamlet talks about the "pales and forts of reason." He is using a military allusion to continue the mole metaphor: The tunnels, "o'ergrowth," destabilizes the palings--the picket fences--of the fort of the mind, the fort of reason and logic. He is talking about strength of mind and intelligence that can harmed by an undercurrent of innate psychological disturbance that tunnels through a man's reason and logic: "some vicious mole of nature."
Hamlet, first addressing the Ghost, refers to the Ghost's bones as "canonized" because they were once buried in a tomb ("hearsed in death": hearse may be a canopy over a tomb) but have now "burst" from the grave. This is an implied metaphor comparing his father's bones to the holiness of a saint, to one who is canonized and holy. This metaphor is a combination of personification (as if the bones themselves have broken out of the grave) and metaphor. The entire phrase basically says: You were once buried with sanctity and holiness. Why now are you here before me?
Hamlet does not value his life at all ("pin's fee"), or my life is not worth more than a pin, therefore he is unafraid to go with the Ghost. This metaphor compares Hamlet's life to a sewing pin: not worth much.
Hamlet offers more metaphors to aid in describing the type of man he means that has "vicious moles":
...that these men,
Carrying, I say, the stamp of one defect,
Being Nature's livery or Fortune's star,
His virtues else, be they as pure as grace...
Here, the metaphor compares the "mole" of defect to (1) livery worn by servants to identify the noble or royal house to whom they belong and to (2) the chance of the astrological sign one is born under. Hamlet is saying that the "defect" is something a man wears (his "livery), bestowed upon him by "Nature" or given him by "Fortune," suggesting that a man's defect may come naturally by family or be a part of his astrological destiny.
1. (vv26-27) Hamlet speaking: By the o'ergrowth of some complexion, oft breaking down the pales and forts of reason...
Metaphor that transforms the human body in a citadel where reason is kept. Shakespeare often refers, in this drama, the human microcosm to a citadel or a castle.
2.(vv 36-37) Hamlet speaking: The dram of evil doth all the noble substance of a doubt to his own scandal
Evil spreads like an infection. Like the human body, even the society could be corrupted by an infection due to a mistake of nature or a dissolute tradition or a human sin.
3.(vv 69-70) Horatio speaking: What if it tempt you toward the flood, my lord, or to the dreadful summit of the cliff
The vertigo of the cliff is a metaphor of the reason that Hamlet could loose following the ghost. Hamlet could go haywire trying to understand the supernatural.
4.The Nemean Lion- (vv 82-85) Hamlet speaking: My fate cries out, and makes each petty artere in this body as hardy as the Nemean lion's nerve.
The Nemean lion is a monster in Greek mythology whose fur was invulnarable. Hamlet warns that nobody could stop him to follow the ghost.