At the east of the map, in the wooden structure east of the room, climb onto the eastern window's edge, and then run at an angle toward the platform on the east, where you can acquire a box of shells and a berserk pack. (sector 10) The jump is a little tricky, so it may take some practice.
After the yellow skull key door, open the door in the next room. Pass under the doorway you came through, and a wall will lower in the westernmost room. Run back in and dash into the newly revealed compartment before it closes to get a backpack. (sector 136)
Upon grabbing the red keycard, return to where you found the yellow skull key to find that a teleporter has been revealed, leading back outside. This actually consists of two sectors, both of which are marked as secret. (sector 159)
See secret #3. (sector 141)
The map name is an obvious allusion to Stonehenge, a place in England containing a circle of stones, considered a sacred place by druid/celtic cultures of ancient Europe.