Spell description : "Caster creates a dome of force around a given point (which may be on a mobile object) to protect an area. This force shield can sustain 10 hits per level of the caster before being destroyed. Otherwise it must be dispelled (or its duration must expire)." AoE 10' radius, duration 5 min/L (C), Range T.
Note from p25 of MentCo : "Seal Breach, Mantlet, Palisade,and Bastion spells are impassable (until destroyed or terminated) from the outside to the inside only. Attacks and exit may still be made from the inside to the outside. All defenses created by these spells must rest on a solid surface. All attacks against these barriers should be made against AT 20 (with a +0 DB)."
The spell is ArmsMaster Fortress Law Lvl 7. Mantlet and Mantlet True are explicitly exempted from the "defend a fortress" general condition of the spell list.
So, the Armsmaster has an impassable field that can absorb 10 hits/level (at least 70) and is considered AT 20. The Armsmaster must concentrate, but allies inside the radius do not need to and may attack from inside the field. Furthermore, the field may be set on a mobile object. If the Armsmaster is high enough level, they can cast Mantlet True - and be able to attack since Mantlet True does not require concentration (in addition to being larger).
How do you handle criticals against the mantlet field ? Do they inflict additional hits merely by virtue of being criticals, or do you apply the strict critical result, meaning that, if the crit inflicts additional hits, it counts against the mantlet buffer, both any other result is voided ?