the mountain is a new area to the smurf village which is accessed after you build the hot air balloon. At level 26
Once you get to the mountain, you will meet grandpa smurf , who will then give you a quest to dig up his glasses. He will give more quests after completion. You start off with 3 smurfs.
In the special area of the build menu on the mountain, you can buy for 200 gold a normal shovel to dig for items on the mountain, or use 2 smurfberries for a gold shovel. A normal shovel takes 30 minutes to complete, while a gold one takes 5 seconds.
Some if the wonders you can build are : the smurfy blimp, the observertory, the castle, the windmill and more.