When you first arrive in Venice, you will meet your contact Dr. Schneider and be brought to the library. In here, you will search for the entrance to catacombs, and optionally some books. The library is divided into sections, alternating between a window and two sets of shelves.
To find the books, you will need to select "what is", and move the cursor on the shelves until you see the word book (instead of books). As finding the books is more of a pixel hunt in addition to needing to scour multiple rooms, the three important ones will appear in fixes locations, best found by moving left:
- How to fly a byplane. One shelf room left of the entrance, the bottom shelf room. It's used as an alternative to the Zepplin, and is optional.
- Mein Kampf, found two shelf rooms left of the entrance, in the bottom shelf room, and on the right. It's used for bypassing guards later.
- Secrets of Roman Catacombs, found three shelf rooms right of the entrance, in the bottom shelf room, and on the right. It shows the map of two floors of the catacomb maze, and is optional if you copied tha map elsewhere.
For the catacombs, you will first need to find some items. One of the window rooms will have a metal post and a red cordon. Pick both of them up. Next, use the grail diary - you should see a stained glass window and some text. Find the window room that matches what's in the diary, making sure angels are facing the correct direction and the image above the shield matches. The text should say something similar to "third on the right"; each window has two pillars to the side with three roman numerals. The left or right indicates which pillar to read, and the number indicates which of the numerals is referenced. Use the metal post on the matching roman numeral to break the floor and enter the catacombs.
If you selected the wrong tile, you can get another metal post to retry.
Catacombs - first floor
When you land in the catacombs, you will have three doors, which basically lead to the same corridor. If you have one of the books from the library, you can read it to get information on where to go, otherwise you'll have to search for the rooms themselves.
The first room is located south-east of the entrance. It has three pirates and a hook, which you should take.
The second room has a torch, but there's mud that's caked on and hold it in place. Continue onward.
The third room has just a stone slab, which is a one way door from below.
To the east and south is a room filled with water, which you need a bottle to collect.
The fifth room is east and north from the third, and it has a manhole leading back to the Venice restaurant. To get a bottle, head to the left and look at the wine bottle to learn that it's a dreadful year, then you can take it and head back to the catacomb. With the water bottle, head to the water room and fill up the bottle, then head to the torch room to loosen the torch. Pull the torch and you'll fall into a trap door.
When you land in the lower floor, head to the north-east, and follow the path all the way to the right. Cross the bridge, and you'll reach a small room with inscriptions. Read the inscriptions to learn what the grail looks like - either write them down or keep a saved game to reference.
Return to the bridge room. Put the hook into the plug, and pull it with the whip. This will drain the water, allowing you to reach the rest of the catacombs (but blocks off the inscription room). To return to the first floor, exit, and take the left path from the fork to reach a ladder.
Head through the water room, and take the first north-west branch. This machine room is used to operate a drawbridge later, thus put the cordon on it, and turn the wheel to have it lowered.
Continue to the north-east corner for the final room on the floor. The room has three movable plaques, and has the solution written in the grail diary. To solve the puzzle, you need to spin the plaques in the correct order - first get the one on the right to match, then the left-hand plaque, followed by the middle. If you need to defend against the "certain death", you can move the middle plaque so that it has anything other than the bad option. When all three plaques are correct, you can proceed to the next floor.
Catacombs - second floor
On the second floor, head to the left to the drawbridge room. If it's still raised, backtrack to the machine room found on the first floor, and turn the wheel.
Follow the north edge until you reach a room with a skull xylophone. Read the diary again for the solution - there are five notes which can be in six positions. The upper-most line in the staff is the highest note on the xylophone, with the next line representing the next note below. Because Indiana is facing the camera when using the skull xylophone, the highest note is on the left. Enter in the five notes - if correct, the door will open.
Catacombs - third floor
The third floor of the catacombs isn't mapped, but is a simple maze. With each vertical branch, you want to take the upper path, lower, lower, middle and lower.
The end will bring you to the knight's room. Open the casket and the inscriptions there will give you the destination of the Grail's temple. You can unlock the grate by pulling on the rusted lock, and exit through the manhole.
When you exit, you'll learn about where your father is located, and will take a short diversion to Castle Brunwald.