Town Hall Grid?
Solution 1:
Source
When a level 10 Town Hall is destroyed, the ruins are lying on a grate on top of lava.
Also, it is stated that
The lava in the level 10 Town Hall may be a reference to the fact that the Inferno Towers are unlocked at this level.
So, basically it is a metal grate (probably on which the town hall is placed) and below it there is lava.