1. If I am on the Portal of Power at the start of my turn, can I try to open it?

2. If I fail in opening the Portal of Power, can I try to open it immediately in my next turn?

Rimmer said:

1. You roll for your move (obviously you'll score at least a 1, which is enough to reach Plain of Peril), then if you wish you can try to get in. Only having an active Warlock Quest prevents you from opening the portal. The portal lies between Plain of Peril and Portal of Power Space, so you try to open it by moving between these spaces (same happens for the Sentinel).

2. Yes, but remember that a failure at the Portal of Power makes you lose 1 point of Strength or Craft, depending on which value you chose for the check.

What if there is a card on the Portal? Do I encounter it again after my first failure?

Rimmer said:

Rulebook pag. 19

A character attempting to open the Portal follows the instructions on the Portal of Power space. If successful, the character’s turn ends on the Plain of Peril. If unsuccessful, the character’s turn ends immediately on the Portal of Power space.

Failure = turn ends immediately. No encounters

Success = turn ends on the Plain of Peril Space, but you can encounter the space. This means that Characters or cards on the Plain of Peril may be encountered.

Thank you very much! Very clear and helpful.

I assume it's pretty the same with the Sentinel.

Rimmer said:

Yes, look at page 18, it's exactly the same. If you're defeated or have a stand-off, your turn ends immediately and you stay on the Sentinel Space.

It's nice to be helpful.