Rules As Written seem to indicate that spells ALWAYS have subtypes and you don't have to prove those subtypes in order to be able to cast the spell (Cassiel, Gurmash, Serseiryx).
I don't like this view at all.
Cards always have a subtype, true.
But the examples provided are all familiars that have no option but to cast certain subtypes of spell. When Gurmash casts a spell you know it has to be a command spell. There is no doubt about it, so nobody should expect to ask for, or show, proof of the subtype.
The Mage, on the other hand, can cast anything. You cannot assume that the spell is of any subtype, and while you could ask for proof, you cannot expect to be shown any. The whole point of face down enchantments cast by a mage is the "it could be anything!" factor, after all.
I argue that until it is revealed, a face down enchantment's subtype is an accounting matter only, useful only to delineate who can cast it.
From the rulebook about enchantments:
"When you cast this spell, it creates a magical "matrix" on the target.
At this point, it is not fully formed and exists only as magical potential, with no effect on the battle. When the spell is revealed later, the caster "completes" the spell by adding additional power.
It is only then that the enchantment takes form and can affect the target." (my emphasis)
This suggests strongly to me that a face down enchantment is nothing. It is not a command spell, it is not a healing spell. It becomes these things only when they are flipped face up and the word 'Command' or 'Healing' is visible to all.
Until it is revealed, an enchantment is a Schrodinger's Enchantment. It is a command spell and a healing spell and a curse (and so on) all at the same time, and yet not one of these things. Only when observed is its actual state confirmed.
To suggest that a face down enchantment can be said to be any specific subtype is unprovable until it is revealed. And if you cannot prove it you can't apply a discount to it, because unlike a familiar-cast spell, it could potentially be anything.
I say the Cape should not be able to provide a discount to a face down enchantment.
I'll use another counter example to try to prove why it would be silly to give face down enchantments a subtype.
A creature has Mind Shield on it.
The opponent's Naiya casts a face down enchantment on that creature.
The owner of the Mind Shield reveals it during the Counterspell step and removes the new enchantment, on the grounds that since naiya cast the spell it is clearly a song, therefore of the psychic subtype, which is affected by the Mind Shield.
The Naiya owner points out that the spell could technically be a Healing Madrigal, and therefore not psychic and not affected by the Mind Shield. But he won't flip the card to prove it.
Now, who in this situation has the right of it? Clearly the Naiya owner has the advantage, because of the inherent uncertainty about what the spell is. 99.99999% of the time it will not be a Madrigal and instead a psychic song of some sort, but until it can demonstrably be shown to be a psychic spell the Mind Shield has to remain inert.
Like the Mind Shield, the commander's cape must remain inert until the enchantment is revealed, becasue the person wanting to use the power cannot state with certainty that the enchantment is a Command.
Sorry for the big rant about what seems a teeny tiny point, but I think it warrants the discussion.