The spell system now supports cast durations longer than the normal current turn (like 3 turns or whatever you deem appropriate for the spell in question). It also grants attacks of opportunity for all creatures who are standing adjacent to a caster while the caster, aeh yes, casts

There is only one interruption save per round, but it increases in difficulty for all damage taken. The DC is 10 + all damage taken (will save). Spellpoints (or hp cost) are lost anyway, even in case of interrupted cast attempts.
You can design individual spells to be non-interruptible or not to trigger AoO, too.
Furthermore, there are (this is not new, but was added by Jer already some time ago) spells that do not even require the current turn, ie they can be cast and the caster can still continue its current turn.These are always non-interruptible and do not trigger AoO.
Finally, two new effects have been added that - when acquired - allow the recipient to become a non-interruptible caster or one that does not Trigger AoO while casting (or both).
In detail:
2. New spell properties:
triggersAoO: When set to true, a player caster with an enemy in melee range receives an immediate free attack from this enemy. This will repeat during each rund of cast duration.
canBeInterrupted: When taking damage while casting the caster has to make a will save with DC 10 + all damage taken during the round or the spell is lost. Th espell cost in sp (or hp) has to be paid nonetheless.
castTimeInTurns: The number of additional turns needed for casting. Default is 0, meaning the spell just uses the current action and no extra turns.
usesTurnToActivate: When set to false, the casting does not even consume an action. The player caster can still act fully after casting such spell, like attacking or casting another spell within the same turn..
3.New effects:
allowCastingWithoutRiskOfInterruption: This effects allows - while it lasts - to cast any spell without risk of being interrupted due to damage taken during the castung process. Authors can e.g. connect it with a trait as permanent effect and create a player class this way that does not suffer from interruption risk.
allowCastingWithoutTriggeringAoO: Casters under this effect will not trigger attack sof opportunity due to their casting.