These three trails are short hikes which lead from the same parking lot on the east side of the park, just north of the visitor center. The Window "Trail" is a boardwalk leading to a nice view, and the round trip is only a quarter mile. The other two, however, are more interesting.
I love this ominous view of the start of the Notch trail. The warning on the sign, the trail winding back towards the towering badlands wall, and the angry, stormy sky. A furious, hailing thunderstorm had just passed. Yes, in July. The trail to the Notch is not long (about a mile and a half roundtrip) but packs in a lot of scenery. After walking through a canyon, I climbed up a sort of a log staircase / ladder (it's hard to describe) onto the badlands formations. From here, the landscape became more alien.
Here, I'm looking back down the canyon I hiked up. To the left, you can see the ladder / staircase and even a few people on the top of it:
At the end of the path, there is a notch in the wall, and you can look through over the landscape surrounding the park:
The Door Trail begins on a boardwalk which leads through a gap (the eponymous Door) in the badlands wall, allowing a view into a plain surrounded by craggy walls and crisscrossed with jagged canyons. You can walk off the end of the boardwalk and continue across the plain - but be careful of the fissures!
I guess they don't want you going beyond this point, which would mean dropping into a canyon:
The Door trail is only about a mile roundtrip. The Door and Notch trails pack a lot of cool scenery into a very short pair of hikes. If you only have a short time in the park, these are more than worth your while!
Castle Trail (and Saddle Pass and Medicine Root Trails)
The Castle Trail is the Badlands' only trail over 2 miles in length. It is, in fact, 5 miles from one end to the other, and of course 10 miles to go out and back. I hiked from the east end to the middle and back on one trip, looping back on Medicine Root Trail, then on a later visit climbed up Saddle Pass and hiked the western half. The trail winds lazily along the base of the huge, intricate badlands wall, which does indeed look like an endless castle (this photo is from the eastern part, which is the more scenic):
Castle Trail is one of the most magnificently sublime experiences I've had out on the trail. Both times I hiked on this trail, the temperature of an otherwise cool day roared up into the upper 90s and lower 100s. I floated along the rough trail almost effortlessly, basking in the glorious, ever changing badlands walls and the ferocious heat. The experience of this hike is hard to describe, but it's one of those places that fills me with a sort of non-religious reverence. This path doesn't follow a proper trail, but instead is marked by thin signposts. (You can make one out in the bottom middle of the above photo.) The eastern half of the trail is more spectacular as it meanders along the base of the badlands wall.
The trail is almost entirely flat, but the ground is broken up and full of little fissures and cracks. Watch your step, and watch out for snakes! (MST3K reference aside, there are rattlesnakes here, so keep your eyes open.)
In the lovely grassy prairies, unfamiliar, colorful birds are everywhere.
As comfortable as I was on this trail, I didn't risk hiking the whole 10 miles on a ferociously hot day on a trail with no shade. But I did loop back to the eastern trailhead via Medicine Root trail, which heads away from the wall into the grasslands. The views aren't as shockingly cool as Castle Trail's wall, but the cactuses, flowers, birds, and butterflies are nice.
The next time I hiked Castle Trail to complete the western half, I climbed up Saddle Pass, a steep, rocky, quarter-mile climb. Here it is from the top:
Note the vehicles in the parking lot below! Saddle Pass puts you in the middle of Castle Trail, right where Medicine Root splits off. The western side is also nice, but not as incredible as the eastern side. This is a scene near the western trailhead:
At the western trailhead, I took a quick walk around the Fossil Exhibit Trail, a short boardwalk which does exactly what its name describes. This was another extremely hot day and I was glad that I could end the hike by heading DOWN the Saddle Pass Trail (not up)!
These trails (plus the Cliff Shelf Nature Trail, another short trail on a boardwalk) are Badlands NP's entire trail system. There's not a huge amount of hiking, and they can all be done in a single day if you avoid a heat wave or a thunderstorm. But it's worth a trip all the way out to the Badlands just for the Notch, the Castle Trail, and the incredible drive around the park.