If you didn't open the gate between the TEMPLE OF POSEIDON and the COASTAL RUINS (C on the map of the ruins), you'll need to take a different route to get back to the LOST LIBRARY and/or HALL OF DEMETRIUS.

After sliding back into the COASTAL RUINS from PHAROS, TEMPLE OF ISIS (through the chute marked G on the map of the ruins), climb out of the water on the right (west) side of the pool and go up the ramp on the side of the building.

The gate here is marked B on the map of the ruins. If you opened this gate earlier, you can go inside and follow the passageway back into the CATACOMBS.

NOTE: If you didn't open this gate earlier from the other side, see this separate page with second alternate route.

When you re-enter the CATACOMBS through gate B, you climb down a ladder into the ROOM WITH SERPENT CARVINGS AND PEDESTAL, where you found TRIDENT #4 earlier. Go to the far end of the room, drop down through the opening in the floor, and then either drop down again or slide down the climbing pole.

At the base of the pole, drop down twice more to the LOWER LEVEL of the
ROOM WITH CIRCULAR SKYLIGHTS.

Head for the southeast corner of this large, square room, where you'll find a doorway leading into the long, north-south aisle dividing this area. Turn right into that aisle.

Then turn right again and climb the stairs...

...to get back to the HUGE ROOM WITH HIGH PLATFORM, ROPES, AND SMALL POOL. Exit through the large doorway near the pool to enter the TEMPLE OF POSEIDON.

At the top of the stairs, climb down the ladder opposite the alcove with the golden bird statue to reach the JUNCTION below.

This is the area where you activated the 4 POSEIDON SATUES earlier, diverting water into the deep pit. Dive into the flooded pit.

Swim down through the passage on the west wall...

...then through the narrow opening at the bottom of that passage on the left.

This leads to a room where you can climb out of the water. Follow the stairs up to the left.

This leads to the ROOM WITH THE FOUR SMALL POOLS AND THE FACE MOSAIC, where you found the LEFT GAUNTLET of the ARMOR OF HORUS earlier.

Go through either of the small doors flanking the sarcophagus.

Follow the passageway around the next corner. There's a raised crawlspace about halfway along the passageway on the west side. Climb into it.

Crawl through, and continue to the LOST LIBRARY.

If you have not yet completed the LOST LIBRARY and HALL OF DEMETRIUS, follow the LOST LIBRARY walkthrough from the beginning to get the PHAROS PILLAR. Then continue to the next level, HALL OF DEMETRIUS to get the PHAROS KNOT.

If you have completed the LOST LIBRARY and HALL OF DEMETRIUS but missed the PHAROS PILLAR, check the section near the end of the LOST LIBRARY walkthrough on returning to the upper level in order to find the artifact. Then retrace the route shown above to return through the crawlspace back to the TEMPLE OF POSEIDON. Follow the POSEIDON walkthrough to open gate C and return to the COASTAL RUINS.

If you have completed the LOST LIBRARY and HALL OF DEMETRIUS but missed the PHAROS KNOT, go through the big doors at the east end of the main hall in the the LOST LIBRARY (shown in the screenshot above) to re-enter the HALL OF DEMETRIUS. Follow the DEMETRIUS walkthrough to find the artifact and exit back to the COASTAL RUINS.

Once you have both artifacts, re-enter PHAROS TEMPLE OF ISIS through the underwater passageway at F on the map of the ruins.

[Pharos Walkthrough]