Wall Traps

Level 8+ walls would contain features upon their destruction:
Level 8- Since these are skull walls they would have a 50% chance to spawn a skeleton when they are destroyed
Level 9- Since these walls have spikes on the side they will send shards in a small area, damaging all nearby troops
Level 10- Since these walls are "lava" walls they will spew lava when destroyed, damaging over time in a small area (possibly negating the heal spell like the infernos)
Level 11- Since these walls are electricity, they will send their current to all troops touching a wall plus all troops nearby other electrocuted troops (also negating the heal spell possibly)