Still spell with Arcane Spell Failure and Mounted casting

Question

My players asked a couple of questions concerning the Still Spell feat.

First, on armor:

Arcane Spell Failure: Armor interferes with the gestures that a spellcaster must make to cast an arcane spell that has a somatic component.

Ok, that's pretty clear. But does that really mean that a Wizard can be in a full-plate and be able to cast a spell with no somatic component? There's no other limit to it?

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Second, on combat while on horseback:

Casting Spells while Mounted: [If] you’re casting the spell while the mount is moving [then] you have to make a Concentration check [...] (DC 10 + spell level) or lose the spell. [etc]

Would still spell make the concentration check easier? Or it is already assumed that the somatic part of the spell should be considered separately? (ie: no ride roll needed but concentration stays the same)

Answer 1

Does that really mean that a Wizard can be in a full-plate and be able to cast a spell with no somatic component? There's no other limit to it?

As far as a wizard wearing full plate, there would be no inhibition on spellcasting other than stilling a spell and having the necessary higher level spell slot to do it. Note however that there are other stipulations to wearing armor one is not proficient in, and there isn't a spell that I know of which grants temporary proficiency to armor specifically.

Pertinent is this rule about armor proficiency:

A character who is wearing armor with which she is not proficient applies its armor check penalty to attack rolls and to all skill checks that involve moving, including Ride.

And including concentration. Just in case you want your wizard in full plate on top of the mount.

Would still spell make the concentration check easier? Or it is already assumed that the somatic part of the spell should be considered separately? (ie: no ride roll needed but concentration stays the same)

I would rule that it would not, agreeing with KRyan's answer as far as that goes. Just like concentrating in a swarm or other distracting conditions, it seems the rule for mounted spellcasting is derived from the same. Concentration check still stands.

Answer 2

Arcane Spell Failure

Arcane Spell Failure only applies to Arcane spells that have Somatic components, yes. Wearing any armor, even armor you are not proficient in, has no effect on spells that are not Arcane or Arcane spells that do not have a Somatic component.

Still Spell is a (very expensive) way around Arcane Spell Failure, yes.

There are far, far cheaper ways. A +1 twilight¹ mithral² feycraft³ githcraft⁴ full-plate with thistle-down padding⁵ has a 0% Arcane Spell Failure for just money, specifically 15,600 gp. Saving yourself a feat and a +1-spell-level cost on most of your spells is easily worth that much.

Using mage armor or a Chain Shirt (+1 twilight mithral, mithral fey-or-githcraft with thistledown, etc) of some kind would be much wiser, though, since it is a lot of money. AC is not that valuable for most spellcasters. A dedicated gish (warrior-mage) probably wants levels in prestige classes that mitigate or ignore Arcane Spell Failure anyway.

¹ Magic Item Compendium, reduces Arcane Spell Failure by 10%. +1 equivalent, magic.

² SRD, reduces Arcane Spell Failure by 10% among other things. 9,000 gp for Heavy Armor, mundane.

³ Dungeon Master’s Guide II, reduces Arcane Spell Failure by 5% among other things. 500 gp, mundane.

⁴ Dungeon Master’s Guide II, reduces Arcane Spell Failure by 5% among other things. 600 gp, mundane.

⁵ Races of the Wild, reduces Arcane Spell Failure by 5% among other things. 250 gp, mundane.

Combining feycraft and githcraft requires either a team effort by a fey armorer and a gith armorer, or some kind of weird fey-gith hybrid armorer. No rules explicitly detail how to combine these two, but no rule restricts them from being applied to the same armor either. If the combination is unavailable, a single level of Spellsword (Complete Warrior) can handle the remaining Arcane Spell Failure for relatively low cost.

But I would strongly recommend not just allowing the combination, but making it a plot point. A great hero needs an incredible suit of armor, needs to draw upon the skills of not just a master armorer, but multiple master armorers from several completely unrelated races? That sounds like a pretty epic little side-quest. Depends on the campaign, of course.

Mounted Spellcasting

This has nothing to do with gestures, and everything to do with not losing your focus on the spell. Still Spell has no affect on it.

The Concentration check should be very easy for most mid-level spellcasters. With their low HD and little need for ability scores other than the one used to cast spells, most spellcasters should make Constitution their second-highest ability score, which means starting probably with 14, and picking up a +2 Constitution item sooner rather than later (though after the +2 Int/Wis/Cha item, obviously).

By level 5, you have 8 ranks, should have at least +2 from Constitution, for +10 to begin with. You automatically succeed, then, on 0th- and 1st-level spells, and only need to roll a 2 for 2nd-level spells (95% chance of success) and a 3 for 3rd-level spells (90% chance of success). By 7th level, with 10 ranks, you have a 95% chance of succeeding on 4th-level spells. By 9th, you can automatically succeed in casting any spell you are able to cast.