Portal is a very good way of taking out enemy buildings, and since that is so important in this game i think one flux is a little cheap. This can be fixed by making it more expensive or to make buildings illegal targets. I think the last suggestion is best. It wouls still be a cool card to ship in troops with.

On the other hand, the building returns to the original place every 2nd turn, and then you just put another building next to it, and - suddenly you have reach to another space as well! (Possibly well, especially with 2 visioned buildings.)
And used for creatures - your enemy can use it to send his creatures to your base too, and there's no way to prevent him from doing that (unless you put there your creature, which'd be out of fight for next turn.. and if he starts 1st the turn after he gets to you anyway.)
So.. I considering all this, I don't think it's that strong..
(Of course goes well with Loxlorn!)

belive me it is that strong. It is like take out one of your oponents buildings + take over his flux well for one flux. I've played it, and it wrecks havoc. Absolutly too strong. One should not be able to target occupied fields.