Field Barrier
Continuous Spell
Field Spell Cards cannot be destroyed. Also, a new Field Spell Card cannot be activated. You can only control 1 face-up "Field Barrier" at a time.
A new Field Spell Card cannot be activated. Simple as that. It doesn't matter what else is on the field. You can Set Field Spell Cards, but you cannot activate any.
You cannot Set a Field Spell Card if you already have a Field Spell Card (face-up or face-down), because to do so would mean you had to destroy your first Field Spell Card (by game mechanic - you only have 1 Field Spell Card Zone). Field Barrier prevents Field Spell Cards from being destroyed, even by doing this. Therefore you can't replace your Field Spell Card with another by any means.
Field Spell Cards cannot be destroyed, but they CAN be sent places (e.g. Giant Trunade, Emergency Provisions).
Basically, Field Barrier locks up the Field Spell Cards on the field. The only thing a player could do with them is Set another one, provided their Field Spell Card Zone was empty; or they could send one somewhere.