Main Portals & The Haven Isle
There are 2 sets of Main Portals that can be accessed in both Single Player and Multi-Player, the only exception is that The Haven Isle cannot be reached through the Portals in Single Player:
There is also a Portal Room under The Haven Isle if you are playing in Multi-Player, the location that each room brings you is marked below:
If you speak to the goblin who is standing in the center of the Portal Room he will give you a warning:
As Anur says... THESE ARE ONE WAY PORTALS! Once you enter one you cannot come back to The Haven Isle without walking to a Main Portal.
The Haven Isle is just East of Silver Creek, it is so close that it can be seen if you are standing outside of the merchant's building, it cannot be reach from town though. If you are playing in Single Player you can use a Combat Jump or Phase Shift to reach it if you really want to (by going East then South around the bay and jumping from beneath the Isle) but there is really no point. In Single Player there is only the Portal that leads to Bellevue, your stash, the Merchant, Combo Master and Blacksmith, none of The Haven Isle's other features are available. If you are playing in Multi-player however The Haven Isle has many uses... First off this is where you appear when you enter a game or where you return when you die within the game. There is of course the Merchant, Combo Master, Blacksmith and your stash all on the Isle. If you go down the stairs in the small tower you will enter the Portal Room and lastly there are three additional Portals on the surface of the Isle. They are marked above... the first brings you to Bellevue, the second brings you to the exact location that you last died and the last brings you to the Dwarven Ruins right next to where the Underworld Campaign begins.
There are also a number of "mini-portals" throughout the game, most having to do with a Main or Side quests. You usually need to obtain a "Portal Rune" to activate them.