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
F7 switch to zonemap mode
F8 show settings menu

Use the arrow keys or RIGHT CLICK and drag to move around the map.

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

Use ALT+arrow keys to move the view of the map.

While you are in passability mode, you will see thick flashing lines that indicate where walls are located. (You can customise how these look in the Settings.)

At the top right corner the current drawing tool is shown. The tools are:

D - Draw tool:
  Press one of the keys to modify
  walls that are described below (see
  "Drawing Keys" below), or press and
  hold down SPACE while moving the
  cursor to draw blocks of walls in a
  line.  You can also use the mouse:
  either LEFT CLICK to add or remove
  one wall, or DRAG the mouse to draw
  individual walls in a line.  If you
  hold down space or drag the mouse,
  then the tile you start dragging on
  determines whether you add or
  remove walls.  You can hold CTRL
  while clicking or dragging to add
  or remove blocks of walls (all four
  sides at once).

S - Stamp tool:
  The stamp tool is like the draw
  tool (it draws on one tile at a
  time) but is for adding or removing
  just one wall type. It can also add
  or remove all walls to/from a tile.
  First, select which wall bit you
  want to draw, either by pressing
  one of the drawing keys (see
  "Drawing Keys" below), including
  DELETE for all walls, or with the
  MOUSE WHEEL.  Then press SPACE or
  CLICK (and DRAG) on a tile to to
  add that wall, or remove it if the
  tile already had that wall.  You
  can hold SPACE and move, or DRAG
  the mouse to change a whole line of
  tiles.

B - Box tool:
  Press SPACE/CLICK once to place one
  corner of a rectangle, move the
  cursor to the opposite corner, and
  then press one of the Drawing Keys
  to modify the walls for all tiles
  in the box.

P - Paint on Layer # tool:
  Finds the continuous area on the
  current tilemap layer under the
  cursor, and changes the walls for
  all those tiles according to the
  Drawing Key pressed; like a flood
  fill of a map layer except that the
  map layer isn't modified. You can
  use PageDown and PageUp to change
  the selected layer. For example,
  you can use this to cover a puddle
  of acid with harmtiles by pressing
  H.

M - Mark tool:
  Use to select a rectangular of the
  map to copy. Press SPACE/CLICK once
  to place one corner of the
  rectangle, and SPACE/CLICK a second
  time at the opposite corner. The
  tool will then be changed to the
  Clone tool. Mark copies the visible
  tilemap layers, wallmap, foemap,
  and zones in the rectangle.

C - Clone tool:
  Press SPACE/CLICK to paste a
  selection copied with the Mark
  tool.  You can switch to different
  editing modes to preview the
  changes to walls, foemap, and
  zones.  If default passability is
  on (press CTRL+D to toggle), the
  wallmap will updated using default
  walls instead of the copied walls.
  Only visible map layers are
  modified (press ALT+~ to toggle
  visibility of the current map
  layer), so a map layer has to be
  visible both when Marking and when
  Cloning.

Drawing Keys:

Press SPACE or CLICK while holding CTRL to make the tile under the cursor fully impassable in all four directions. Press SPACE/CLICK 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.

CLICK or DRAG the mouse near the edge of a tile to make the tile impassable on that side.

Press W to turn all the walls on the current tile into one-way walls. This means that the hero and NPCS can walk through the walls when they leave the tile. If there are no walls on the tile then the one-way bit does nothing, but is still indicated with a small circle.
One-way walls are marked with a special zone, ID 10000, which you can also see and edit in the zone editor.

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

CTRL+A marks the tile as special for Airships (or other vehicle or scripting purposes which use the vehicle A bit).

CTRL+B marks the tile as special for Boats (or other vehicle or scripting purposes which use the vehicle B bit).

O marks the tile as overhead. This is obsolete. You should create a tilemap layer and place it above heroes and NPCs (in the Layers and Tilesets menu) instead if you want make tiles overhead.

DELETE removes all walls and other bits from a tile and sets the current wall for the Stamp tool to All.

Other Controls:

Hold CTRL while using the Draw tool with the mouse to add/remove all four walls at once (if you use keyboard controls, nothing changes).

CTRL+Z undoes a change to the tilemap, wallmap, foemap or zonemap. Changes to NPCs and doors are not undoable. The undo history buffer is near-infinite, but certain things cause it to be cleared: resizing the map, deleting or swapping map layers, erasing map data, and leaving the map editor.

CTRL+Y redoes an undone change.

CTRL+S immediately saves, useful when live-previewing a game.

+ and - change the way that the walls are drawn on the map. You can also adjust this in the Settings menu.

CTRL+W paints the tiles visible in the window/screen using the last wall type you placed.

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

~ shows a preview of the entire map.

CTRL+N toggles whethers NPCs are visible.