I'd like to be able to make my Zealot (which has a ranged attack) to 'cast' (attack) portals on the ground/walls, and not let him target anything else than the environment itself (terrain).
Also, the Portals of course need to stay open.
I'll ask more questions if once you guys helped me on this, I cannot find solutions on my own.

Thanks in advance! <3

Need more information. What is the type of the ability you're making?

For now, I'll make the spells castable as abilities, but eventually, I'll incorporate WASD movement and the cast of the portals will be done with left/right click of the mouse.
There are 2 portals. Lets call them 'A' and 'B'.
Whenever you enter 'A', you get teleported to 'B', and vice versa.
Also, you cannot have either 2 As or 2 Bs, the most recent Portal over-write the older one, which gets deleted.

What else EXCEPT environment is there? It seems like target anything might work.

Also, it's easy to make it abilities, and have the left-click be portal one and right-click be portal two. Clicking those uses the ability. No weapon needed.

Well, just think of Portal (the game), I want it to be as near to it as I it can be.
So, hum, water wouldn't be targetable, same for units, and I think that's it. XD
I know Cent, but I do not want to focus on making a WASD while working on Portals, so I'll leave them as buttons for nowé

Maybe have his ability build units (portals) instead of cast a spell. That way you can only build where no other units/buildings are. Do the build similar to the ravens turrets it can throw down.

But that wouldn't apply to wall, am I right?

There may be a way to set what it can be built on, but I haven't looked into it all that much.

I just tried pathing and couldn't get that to work.

Well, I'm not sure I understand your solution, and I'm pretty sure I wouldn't be able to achieve it.
Any example map?