Arguably the most difficult level in the game outside of the Special World. This fortress is full of spikes. The key to getting through the level is patience. There are 3 sets of spikes. Each set has several safe areas to wait in while the spikes move. Each set is progressively faster.
For the first set, run to the bottom of the 'valley.' If you've hit the green switch palace, grab the feather down here. When the spikes start to rise, follow them up and run through the spiky tube. If you cape-spin while running, you don't need to wait for the rusty spikes to fall, and can take out the baddies.
The second set consists of 3 groups of pillars (1, 1, and 3 pillars wide). To make it, start jumping as they start to rise. Wait in between groups. After completing this, feel free to grab the mushroom available in the yellow switch block.
The final set is the most difficult and consists of several clusters of spikes. Waiting in between each pair of spikes tends to help the timing for the jumps. Start running when you think the spike is about to go up and you should make it.
At the end is a Reznos. Beat him and you'll have the second shortest path to Bowser!