The player can enter the forest from the onset of the game in the Present; however, Crono cannot enter the castle itself. The next time the player visits the forest is in 600 A.D., following the trail of Marle after she was mistaken for Queen Leene. After acquiring Lucca, Crono returns through the forest and heads for the Cathedral; after saving the Queen the party is taken straight back to the castle, bypassing the forest altogether. The party then returns through the forest, heading for Truce Canyon and the Present; Crono takes Marle back to the castle by way of the forest.
A few sequences later, Crono is escaping from his execution; Crono, Marle, and Lucca are cornered almost immediately after entering the forest. They are backed into a small clearing, supposedly trapped. Here, Lucca pulls out the Gate Key and activates the Gate, sending them to 2300 A.D. and escaping any harm. The group can return to the present, but the soldiers have the area blocked off to prevent any escape. After this point in the story, the forest plays no further part in the plot; however, there will be points at which the party must travel straight through the area to get to and from the castle.
A good start for players is to head out to the forest and do a bit of fighting. This is not expected of the player, but the monsters give more TP and Experience Point than fighting Gato. It is entirely possible to use this area to gain enough money to buy overpowered equipment from Porre.
When Frog begins to reminisce about his past while fighting alongside with Cyrus, the past battle with the Naga and with the Frog King are observed to take place in Guardia Forest. The battle with the Naga takes place in the lower left of the area; the Frog King battle takes place in the lower right.
If you return through the gate from the future, you'll notice that there are soldiers blocking your exit. Once you have the Pendant charged after your visit to Zeal, you can use the gate to access the Sealed Chest there.
Usually, if there is a sealed chest in an area accessed in multiple eras, the chest has a normal and upgraded form, depending on what time period you acquire it from. In this case, due to the different locations (at the end of the path to the gate in 600 A.D., and on the screen with the gate in the Present), the items (a Speed Capsule and a Power Ring) are different.
Each time the player returns to Guardia Forest (in Middle Ages only), the rustling bush can be inspected for a free Shelter. The glitch can be exploited by entering and exiting the forest successively.