It does actually depend at what point in the novel you are referring to but generally you can apply the following to your settings:
1) the mountain is where they meet the "beast" - they must face this particular enemy so to speak which of course they are reluctant to do.
The mountain is also the place where they light the fire so it also represents the hope of being rescued (depending on your context) and gives the feeling of having maybe some control (power) over their future - clinging to hope.
2)the forest - actually the forest glade (a place of natural beauty)- is where Jack impales the head of the sow on a stake which symbolises the loss of innocence and the beginning of the power and cruelty that will descend on the tribe.
The forest as a whole represents secrecy and even the unknown.
3)the beach is almost like the Garden of Eden (at the beginning it is like paradise.) It represents freedom - no adults here! It comes to represent a place of safewty - Interestingly the beach is the place where Ralph flees at the end and ironically he finds adults who have come to rescue them.
4) castle Rock becomes Jack's fort once the boys realise that the beast does not live there. It is safe - they could roll rocks off and "kill the enemy." Jack takes total control here so it represents a place of fear for some who are manipulated by Jack.
Hope this helps!
The significance of the mountain in Lord of the Flies shifts as the plot develops. When the novel opens and the three boys first climb the mountain together, they realize that they are, in fact, stranded on an island; at this point, the mountain symbolizes freedom, a complete and utter lack of supervision by adults, and the whole of the island seems to offer the promise of adventure.
Only later at Ralph's insistence, the boys must keep the signal fire lit on the mountain in an attempt to garner the attention of any passing ships. At this point in the story line, the mountain, along with the signal fire, symbolizes the boys' collective effort to regain contact with civilization; keeping that fire lit requires organization and commitment, both of which Jack's hunters ultimately fail.
In chapter six, "Beast from the Air," the mountain also comes to symbolize fear, as Samneric are 'attacked' by the beast while tending the fire on the mountain. Now the mountain looms as a dangerous possibility, where the beast may lurk unchecked, interfering with Ralph's best designs for attempting rescue. Throughout chapter six and seven, the boys organize a search for the beast with the intent of heading toward the mountain, where the beast was last seen by the twins.