Thanks for the feedback!
As worded this is a Reserve, not a Duratio (you probably have in mind that this becomes a Duration after being called, laying in your play area like a Duration card, but technically it is not).
It's a Duration card so it doesn't clean-up on the turn it's called (unlike Coin of the Realm and Royal Carriage which clean up on that turn - this is to provide tracking for whether its protecting you). It's also a Reserve because it goes to the Tavern mat and can be called/discarded from there. Really just semantics, but I think the Duration type here is correct.
The play effect is too strong, cheap cantrip are easy spammable and thus kill all Attacks (even a mere non-terminal like Lighthouse can kill off Attacks).
It's possible that having it be a cantrip is too powerful due to the filtering, but It should probably be non-terminal. It could simply be reversed to have it discard first, then draw and keep the cantrip. Defense in Dominion is pretty weak overall, but I could see it being a cantrip could result in a fair number of people spamming it enough to block all attacks (which still wouldn't be terrible because of the general opportunity cost of doing so).
The call-effect is far too strong, making it a delayed Forum (and, ignorign the defense aspect, arguably better than Dungeon).
Dungeon filters on the turn it's played. This wouldn't — it's worse in that regard. Also, the value of delaying an effect to the next turn devalues it significantly so having it cost less isn't as overpowered as you might think. It may be something that could bump the cost up to 3 if playtesting showed that. Or potentially, as previously stated, discard first then draw.
The call-to-defense thing makes the defense part of this strictly better than Lighthouse or Guardian.
That's the idea. It provides no benefit while on the mat and so you have to weigh the cost of it sitting there versus calling it, unlike Guardian, which you can buy at any time and have it apply immediately instead of having to buy, cycle, and play it before use. Lighthouse gives you 2 coins across 2 turns, so this is different; it could be better sometimes, other times no.
I thought that the idea of a Reserve defense was to have a trade-off between the flexibility of only defending when needed and then only being able to defend once.
The reason I proposed this idea is that the defend-only-once property scales terribly between different numbers of players. You still can only defend on the rounds you need to (which forces you to track your opponents decks more). If this is too powerful, then you could make it not a cantrip in some way, like removing the +1 Card aspect. Possibly make it terminal with some bonus, but I like the idea of not having to take a terminal slot in your turn for defense, which I think would make it too costly to use.
If I were to weaken it, I'd do something more like:
Shield, Duration-style v2Action — Duration — Reserve
Cost:
+1 Action
Put this on your Tavern mat.
—
At the start of your turn you may discard this from your Tavern mat. If you do, discard 2 cards then
+2 Cards.
When another player plays an Attack card, you may first call this to be, until your next turn, unaffected when other players play Attack cards.
I'd love to playtest both.