Thanks all!
And Crazy Ivan: good ideas. In the first variant it would work like the mausoleum. The second could give a sense of Russian roulette to the game

The following occurred to me:
1. Some of the ancient wizards escaped from the cataclysm by taking the portal. Now they come back and find their town destroyed and haunted by marauders (the players). Which makes them very angry. But their forces fade, so every round they'll get less dangerous.
2. The portal is one cause of the cataclysm, opening a door to the world of a non-human people (like the Vadhagh or Eldren by Moorcock) which didn't like the visit and sent a blizzard to close the door. Yet the connection is stil established. So it is the passage to some incredible treasure, but whoever uses it, risks to wake the perils slumbering behind.
3. The portal used to be the magic metro of the city, but now gives some game advantages: you can step through and go to every place you like on the table. Or you can send a spell or an arrow through it, to hit an enemy, that is hiding before your eyes.
4. The portal must first be activated by runes. There a five runes in the game. If a player gains at least two, he can get through and gets the same number of treasure tokens as he has runes. When activated every player can use the portal that way.
5. The portal is activated from outside. For unknown reason some Eldren appear and hunt down the players, trying to kidnap them and disappear with them through the portal (could even be something like in Predator, but without Schwarzenegger

)