Tiles appear in front of the character.
The largest collision mask for a character is 24x32 though it is difficult and annoying to climb down a 24-pixel wide hole (so 32x32 is sort of a minimum size for a place where a character can walk). The two different characters uses different collision masks, and when you transform between the characters - if an overlap occurs with the new mask it is pushed out of the wall until it no longer overlaps. However, if this process fails (if there's no nearby room for the new character), the player will die. This is technically not a bug, I specifically designed this to happen. Designing levels with way too narrow spaces should be avoided though.
Tiles are rendered from left to right, from top to bottom, like letters in a book. The last rendered tile appears on the top. As the idea is to stick to all-black for tilesets that will make it into the main game, this information is only really important if you intend to break out of the limitations and do a tileset for the library only.