How to open door inside temple of snakes, in town of freehaven

Temple of the snake is actually in Blackshire, unless you meant a different dungeon in Freehaven?

There are two switches that open walls. One is in the room where the Gold Dragon was. The other if I'm not mistaken is north-west on the map.

I guess it's the only temple in Free Haven with snakes (and meduza): Temple of the Moon.
To open the gate, you need to touch the different altars named after a stat, in the order that's listed at the top of the gate.

This temple is for the last druid promotion quest, so they made it appropriately difficult.

I think this is the only matching dungeon I can come up with... the only door that you would have any trouble opening in the actual Temple of the Snake, is a secret door.

But you can easily find out about dungeons and quests yourself, here: the-spoiler.com /RPG/New.World.Computing/might..magic.6.1/mm6.html