Thieves Guild Lair need a key

Hi, i am stuck in the thieves guild hideout, I have killed all the thieves and there is a door that I need a key to open. Where can I get the key?!

I believe you mean the door that's meant for the strong disarm and unlock scrolls that the mission giver gave you. Try lockpick if your lockpick skill level is high enough. There's no key.

There is a tapestry you can tap to leave, behind where you entered.