I've gotten myself into the Portal of Natural Disasters but I now seem to be going in circles, trying to find the opening to the sigil stone. Any help, please?

In the side towers, there are gate controls at the top, which open the war gates down on the ground. You're going to have to open a gate, then descend down, and go up the previously inaccessible side tower, in order to eventually get to the level in the sigil tower where you can go higher.

Also, make sure to explore that world. You can find some very interesting rare items there if you look hard enough.

If you have a good acrobatic skill you save and jump down. Usually you don't fall into fire (happened only once) but arrive against walls at the middle part. If you arrive on a path with doors, you'll find easily your way up. If not, save again and jump down once more to land on main floor. Now you can go up and reach the sigil stone. It's the shortest way...

Many thanks.