This is the last stage of World 1. Congrats! You're doing great. This area starts with a horizontal scroll, changes to a vertical scroll, and then goes back to a horizontal scroll. Cross the waterfall to reach the tower. While in the vertical scroll, you can't open the door far below unless you get the key from the top room. Climb all the way up to find the magic key, and drop all the way to the bottom. In the end, you'll run into Mouser—not a friendly guy.
(1) Now you're in Sub-space. See that potion at the foot of the bridge? Grab it and use it on the spot to enter Sub-space to collect a Mushroom. Watch out; from here on you're going to run into many dangerous and troublesome Trouters. It's a good idea to keep your health meter high.
Cross the river. Now you will encounter three consecutive log rides. Prepare yourself for three waterfalls in a row, with plenty of logs to keep you hopping. This is the toughest jumping you have faced yet, and each one is more difficult than the next.
(2) The long haul for life. There is a potion under the log bridge to the right. But if you try to use anywhere in the vicinity, you won't discover any mushrooms. Instead, grab the potion and take it back to the top of the hill by the waterfall. Enter Sub-space here and you'll find a Mushroom as well as five coins. The potion that you collected has one other use in this world.
As you're climbing up the tower, move the first set of Mushroom Blocks and you may uncover a basket with rapidly-changing contents. Throw it to get a random item, ranging from a Bomb to a Heart Turnip, and even a Star. Star power will be very handy when you have to deal with Sparks.
Before you reach the room with the key, there's a new opportunity to discover in this portion of the world. Duck down into the jar on the right side to discover a potion. Carry the potion back outside with you, and drop it down in the vicinity. Step through the door, and you'll find a third hidden mushroom.
(3) First, take the key! This map of this tall tower is split in half, with the top shown to the left, and the bottom shown to the right. You enter map B through the door A which lies roughly in the middle of the tower (visible on the right map.) When you arrive, start by climbing all the way up to the top room, collect the key behind door B, and carefully head back down the way you came. Door C is locked at the bottom of the tower. Be careful as you fall down. Stay to the right, as you will find a bed of spikes on the floor to the left.
(4) Beware of Spark! Although sparks were once an ally of Mario in his battle against Donkey Kong Jr., they are no friend of his in the World of Dreams. You'll find lots of them in map C, and they're all hoping to zap you. Snifits and Tweeters occupy the platforms in the middle of the map. Use the POW block to shake things up. After a corridor of Ninjis, you'll find the Crystal Ball that is usually horded by Birdo right out in the open. Does this mean you won't have to face a boss battle? No such luck.
Blast Mouser! Mouser is the ruling boss of World 1. Defeating him is the first step toward bringing peace back to the World of Dreams. Use some bombs to help Mario and crew get through the wall. Then stand on the floor below Mouser, to the left side, and catch the bombs that Mouser throws. You don't have to press any buttons, just get under the bomb and you'll catch it. Deposit them back on the platform where he is standing before they explode. Catch Mouser three times in the blast, and he's done for.