Hi Davide,
With a magic portal, you may place a meeple on any feature, including a field, but you can't place it on a feature that do already contain a meeple, and also not on a feature that has already been completed.
https://wikicarpedia.com/index.php/The_Princess_and_The_Dragon#2._Placing_a_meeple_4
Yep... The idea behind the magic portal is that you can place a meeple on any feature vacated with the dragon, that includes fields... So whatever evil the dragon does, can be balanced with a magic portal... with limitations of course.
As corinthiens13 mentioned, the feature to be occupied via a magic portal has to be unoccupied and incomplete. You may occupy any feature placed on a tile (see final notes below).
In contrast, flying machines may land a meeple on any feature on a tile but fields, even if it is already occupied. Special areas are off-limits too (see final notes below).
There is a tiny subtlety here: the tile with the magic portal or flying machine cannot be completing the feature you want to occupy (if a tile completes the feature in
1. Placing a tile, you cannot occupy it with a magic portal -or flying machine- in
2. Placing a meeple). Check footnote #13 for magic portals on the same page corinthiens13 mentioned above:
https://wikicarpedia.com/index.php/The_Princess_and_The_Dragon#cite_note-13 Some additional notes on the use of magic portals and flying machines:
* You use a magic portal to land on a claimable feature so you cannot concatenate the use of magic portals and/or flying machines to bend the rules. For example:
- Using a flying machine to land on a magic portal to place a meeple as a farmer
- Using a magic portal to place your meeple on a flying machine to land on an occupied feature
So both magic portals and flying machines can only be used during the turn you placed them.
* If playing with The Phantom and you are placing a wooden meeple and a phantom, only one of them can use the magic portal or flying machine on a tile.
* Magic portals and flying machines allow you to occupy a claimable feature located on tiles, but not special areas. This means no Exp. 8 castles, no School, no Leipzig quarter, no inside of Carcassonne, no crown space on the Wheel of Fortune. However, you can use magic portals and flying machines to occupy features on the tiles outside these special areas.
* There are special areas on tiles too than cannot be accessed via a magic portal or a flying machine:
- Towers floors (you can place a meeple directly on a tower)
- Areas where meeples stay trapped such as bathhouses, the Solovei Razboinyk tree and the Vodyanoy lake.
EDIT: Added bullet on special areas on tiles that cannot be accessed by a magic portal or a flying machine.