Hidden Treasures and
Fortress of Antonia are both optional automatic triggers. Whichever is active the longest goes first.
I've been meaning to try and hash out an order of operations for this sort of thing, so here goes:
0. Conditions update constantly (such as the X in an ability or a "While X is true" ability like on
Iron Pan) and insert themselves in-between abilities. Always have the precedence.
1. Any ability on a card being played complete.
2. Any ability on a card played due to the initial card being played (i.e. play or band abilities, or weapons) complete.
3. Any mandatory automatic triggers ("When X do Y") complete.
4. Any optional automatic triggers ("When X you
may Y") complete.
5. Any manual activations ("Once per turn you may..." "When you choose you may..." "You may discard this card to X..." and so on) can be activated.
In case of a tie at
any step (except 0, those always happen 'continuously'):
A. If the 'tie' comes from multiple cards
played at the same time, the person whose ability played them (i.e. a massive band) chooses the order.
B. Otherwise, longest-active card wins.
The case you mentioned is a tie for 4 (scenario 4B). Whichever has been face-up on the field the longest continuously, without being negated, may be used first by their controller.
