The door of levels 999 can be found on Deeper Banuta, right here:
To enter on this cave you have 2 ways from the Secret Tunnel of Tiquanda o from the Teleport of Apes. In both cases you have to be done the Ape City Quest at least until the mission 9 for have access of those tp's.
Once you got access to banuta, there will be a wall to you go in:
To unlock it, you will have to use the levers ubicated of the following map with a yellow circle. I would like to recommend you follow the way I marked and use as the last, the lever infront of the wall, because it have time and it can close after some minuts if you dont do it faster.
The monsters that you will face inside Banuta will be:
- Frost Dragon
- Serpent Spawn
- Giant Spider
Once you press all the levers, the wall will disappear and you can enter: