This is the metapuzzle, which means that it will use the answers to the other 6 puzzles. However, the treasures can first be filled in without needing the other puzzle answers. Searching for the three given treasure names reveals that they are all treasures from the first Uncharted game. The silver squares are substituted with the word SILVER and the gold squares are substituted with the word GOLD. The 5 symbols are simple letter transformations that help to fill in the remaining treasures: | = same letter, △ = next letter, ▢ = letter x 2, ⬠ = letter ÷ 2, and ◯ = ROT 13. All of the treasures in the list are among the first 36 treasures found in the game. Furthermore, multiplying the first three given treasures' numbers by 10 matches perfectly with the three numbers in the "wall scratchings" at the top of the puzzle (220 310 330). This suggests that all of the treasure numbers should be multiplied by 10.
Now the previous 6 puzzle answers have to be used somehow: ANGUILLA, LAUREL, ELDORADO, DOGSTAR, ARTHURQWAK, and AKBARKHAN. The last two letters of each answer match up perfectly with the first two letters of another answer, and vice versa. These overlaps can be used to chain the answers together in a 36-letter circular loop. The wall inscription says to "assemble the device that points the way" and the numbers obtained from the treasures look suspiciously like compass headings. Indeed, the 36-letter circle assembled from the puzzle answers is a compass. The compass can be oriented by noticing that there is exactly one instance of each letter N, E, S, and W, and they are evenly spaced around the circle.
With the compass assembled, a letter can be read using each of the headings from the treasures. The resulting letters spell out the answer of how you can get to Avalon: TAKETHEUNDERGROUNDGRAILROAD.