Oh, I see...
Well, there's no invulnerability period. I think this would make
the game just too easy. I agree that sometimes it is unavoidable
to get stunned in some way but the trick is to avoid the chain
reaction somehow and not to drop all the way down.
Lives are only lost when the ball drops to the bottom level so
if you manage to "resurrect" at least at the first level, you
are safe.
You might have noticed that the ball is stunned for a shorter
time when it drops down the hole compared to longer time when
it collides with monster. Sometimes it might be better to just
"commit suicide" by dropping down the hole and come back alive faster
instead of trying to run from the monster, etc.
I know this all can be frustrating at times, especially when the ball drops
to the very bottom from the top line
but my opinion is that this
frustration also makes you return to this game and try to "beat it this time
for sure"
