I'm so sorry I haven't noticed this question earlier. Have you got the key?
It's inside the ruins. Walk sideways along the southernmost wall in the southeastern part of the area you first enter (not the one you can teleport into) until you see a switch. You can also spot the button if you walk south through the 2-step wide hall.
Feel free to ask again if you're still stuck or if nobody answers your question for a while (but please add a lot more details to your question so that it becomes easier to answer). Don't quit until you have tried the Dragonbuilders
Best of luck.