hmm, then you don't know how defense rituals are picked. I would suggest reading the topic dedicated to "Rituals".
The problem is that for low MP levels, it is not easy to have a "self sustained defense ritual".
And if you don't have a valid ritual ... it's better to not have it at all. Read the indicated topic.
What do I mean by "self sustained defense ritual" is that when all it's creatures die, they resurrect with enough VE to keep valid an identical ritual.
That means that you can have (like I said) a ritual die 20 times and still be valid.