Page 11 under Seeing the Target:
To determine if a mouse can see a target for the purpose of a ranged attack, draw an imaginary line from the center of the space the attcking figure is on to the center of the space the targeted figure is on. Use the edge of a card or this rulebook if necessary. If the imaginary line passes through any black areas or walls, then the attack or spell cannot be used. Additionally, there are some board features that may give the impression that they would block line of sight. Players can determine among themselves which of these features should block a mouse's line of sight.