I don't know if it's just me, but I have a real hard time spotting whether crystal walls have a creature trapped inside them or not.
A few ways that it might be possible to make things easier to see:
A) Make crystal wall more translucent so you can actually see the creature trapped inside of it.
B) Change the colour shading of crystal wall to indicate when there is a creature trapped inside of it. (Maybe change by creature type, or to colour of the player who owns that creature).
C) In someway animate the crystal wall to show when it has a creature in it (which doesn't really fit with a crystal wall)