Blocking tiles

tile	walker (1)	swimmer (2)	flyer (4)	magic (8)	examples
F					shallow water
E	X				deep water
D		X			normal ground
C	X	X			abyss; low fence or other chest-high obstacle
B			X		indoor, under overhang
A	X		X		river under bridge
9		X	X		gateway
8	X	X	X		grate, prison bars
7				X	circle of protection on shallow water
6	X			X	circle of protection on deep water
5		X		X	circle of protection on normal ground
4	X	X		X	tree, high stone, other man-high obstacle
3			X	X	?
2	X		X	X	?
1		X	X	X	?
0	X	X	X	X	wall

The column "magic" applies to spells which need a line-of-sight to be cast as well as to "ghost" type mobs / players. A being can have multiple forms of movement, like a harpy which can both walk and fly. These mobs are only blocked when all of their ways of movement are blocked. Or in computer terms:
if ((tile_bitfield & move_bitfield) == 0) tile=BLOCKED;

Having scenarios where players and enemies have different areas to walk on as well as shared areas where both can walk, could lead to some interesting gameplay situations.

I could also imagine some spells which turn a player into a fish, bird or ghost. This would mean that they can't use equipment anymore, but it would allow them some interesting new leveling strategies as well as open up some new areas. There could also be the ability to polymorph a player involuntarily when he is in an area which allows both types of movement. This could be used as a strategy in PvP. Or it could be the solution to some puzzles before the player acquired said spells.

One puzzle I have in mind is a river with a broken bridge. The river is populated by mermaid mobs which sometimes attack the players with a spell which turns them into a fish. To cross the river, the players need to walk into shallow water, provoke the mermaids to hit them with said spell, swim across the river into shallow water on the other side, and wait for the spell to wear off.

An advanced form of magic could allow players to walk on water or fly without sacrificing their ability to walk on land or have equipment. But this should be a lot more expensive for the player.