Spell Rules

According to the Ars Magica FAQ, wards don't keep out a supernatural creature unless they penetrate its Magic Resistance. This is not mentioned explicitly in the rule books but seems to have been assumed by the Fifth Edition author.

One could make an argument either way. The argument that wards need to penetrate is that all spells need to penetrate and there is not an explicit exception for wards. The argument against them needing to penetrate is that the spell descriptions (such as Ward Against Faeries of the Waters, ArM5 p. 124) say how the spells work, and they only mention Level and Might Score, not Penetration.

We think the latter argument is stronger. If wards need to Penetrate, they are seriously under-powered and the spell guidelines should be revised. The lower-impact fix is to play the rules as they were written (though perhaps not as they were meant to be written).