Complex sentence: Sigil cannot be entered or exited save via portals; although this makes it quite safe from any would-be invader, it also makes it a prison of sorts for those not possessing a portal key.
Split into simpler sentences: Sigil cannot be entered or exited save via portals. Although this makes it quite safe from any would-be invader, it also makes it a prison of sorts for those not possessing a portal key.