The map editor is in Passability mode (also known as "Wallmap mode").

F2 switch to picture mode
F3 switch to passability mode
F4 switch to door placement mode
F5 switch to NPC placement mode
F6 switch to foemap mode

Use the arrow keys to move around the map.

Use SHIFT+arrow keys to move around the map faster.

While you are in passability mode, you will see thick flashing lines that indicate where walls are located.

Press SPACE to make the tile under the cursor fully impassable in all four directions. Press SPACE again to remove the walls and make the tile passable.

CTRL+Up makes the selected tile impassable on the top.

CTRL+Down makes the selected tile impassable on the bottom.

CTRL+Left makes the selected tile impassable on the left side.

CTRL+Right makes the selected tile
impassable on the right side.

H marks the tile as a harm tile that can damage the hero when the player steps on it.

A marks the tile as special for Airships (or other vehicle or scripting purposes).

B marks the tile as special for Boats (or other vehicle or scripting purposes).

O marks the tile as overhead. This is obsolete. You should use Layer 2 in Tilemapping mode instead if you want make tiles overhead.

TAB toggles a blue rectangle that shows roughly what area of the map you are currently looking at.