You'll need 120 gems to unlock this level. I have 159.
Toadette finds herself at the base of a tall tower. When you approach the tower, a Magikoopa will appear and blast magic at you. Walk around the tower and you'll find some invisible coins and a pile of coins near the ladder in the back. Walk around the tower and collect as many coins as you can. Use the green pipe in the wall to go up a level. There are some Piranha Plants here. Stand by them to lure the Magikoopa into attacking them. To the left are some Stumpers. Use them to reach the first gem in the alcove and walk to the left. There's a plant near a gap in the floor. It has a key. Toss the key onto the fan, then go through the yellow pipe. Raise the fan and pick up the key. Drop down with the fan to get the coins. Walk to the right to find the locked door with the second gem. Take the red pipe up to the third level. Avoid the Stumpers using an alcove and you'll find a plant with a Mushroom. Walk to the other side and drop down to a P-Switch. This makes the third level rotate. Return to the third level and go through the blue pipe. This takes you to the fourth level. Climb the ladder to the top of the tower. Three Magikoopas will appear and the one following you around will disappear. Quickly run to the handle in the center and pull it for the turnip cannon. Defeat the Magikoopas and you'll be released from the cannon. The center part of the tower will rise, revealing the Star. Before you get it, drop down to the fourth level. Walk to the side of the tower with the Stumpers on the third level and drop down to them. Use them to reach the area inside the tower, where you'll find many coins and the third gem. Collect the Star when you're done.
Special Challenge: Collect 140 coins.