Since this is my first and only Steam guide, this might look a bit unprofessional. Some screenshots are mine and others I found online.
The user victordromano did a great achievement guide (in spanish). It contains all the achievements and some tips, so check it out.
https://steamproxy.com/sharedfiles/filedetails/?id=2560026215

After the two corpses, destroy the fake wall at the left with the emblem. (locomalito's logo! look!)

At the start of the level, go right...



By the way, if you kneel at the statue, you can get an invincibility bonus. There is a lot of secret items and treasures like this in the game.



After the bed, there is a key in the second chest.



Advance through the level until you find this gate.

Ignore the first chain, go up and left in the second chain.



You will find the pedestal at the end.

In the first part of the Aqueduct, after the frogs, you will find this chest guarded with three Enchanted Armors. It's the key.


Pass the elevator. At the end, there is another gate.

After the room with the fireflies, crouch under this tree with the cross.


It will spawn St. Martin statue + an achievement. Shoot it to get the tear.

At this first platform in the library, shoot up.

At the swamp, shoot down in the middle of these dead trees. Summoning the lady of the lake will grant you the fire sword. (Sorry, I lost the frame, lol)

In the final stage, at the castle, three floors down to the right, there is a pillar with a engraved cross. Crouch it to get the note.

Before fighting the giant turtle, grab the Crystal Ball. At the Caverns, go down the first chain and go left. You will find the hidden village

Good Luck at the end stage.
And remember:

For God and Castilla!!!!