Assassin’s Creed IV: Vault Raider

You need to find 16 mayan stones to unlock the door at Tulum (70,405). Here is list where you find all:

Easy
#1 – Cape Bonavista (179, 593)
#2 – Cat Island (742, 695)
#3 – Matanzas (333, 650)
#4 – Great Inagua (845, 469)

Medium
#5 – Tulum (70,405)
#6+7 – Pinos Isle (335,469)
#8 – Tortuga (882, 370)

Hard
#9+10 – Isla Providencia (502,44)
#11+12 – Misteriosa (307,195)
#13+14 – Santanillas (217,250)
#15 – New Bone (422, 118)
#16 – Long Bay (525, 253)

The reward to finding all 16 stones is the Mayan outfit (has the ability to deflect metal projectiles)