So you might be wondering the idea of the portals
They come in different shapes and sizes from man hole covers to actual fantasy portals.
They can be exited but once they do the entities that were left alive will level up and will have extra more entities spawning in the portal.
There are also items that range from support, defense, or attack that are clear or hidden around the portal. There are Tomes that contain different skills or memories know for example on basic spearmenship or trap making or a memory of a person they are also in the portal but are only hidden.
The terrain in the portals will sometimes be based of the culture or region of it for example for a american portal it would be a vibrant city with its surroundings having american flair or it would just be a cave.
Thats all I have to say but I might update this more once I have more ideas