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.