This might be a way of justifying the time spent on my library, but I've also come to realize some interesting aspects of game design that are only possible in action-platform games. I've found that designing maps for sidescrolling games is just more interesting than making maps for top-down games, and I'd like to share some of this with you to hopefully generate some more interest in the library and in action-platform games.
A sidescroller (or platformer, or action-platform game) is a type of 2D game you can make with BYOND. The world is still tile-based, but we're not using the typical top-down view you see in most BYOND games. The player isn't free to move in 4 directions (up, down, left, right), they have gravity pulling them downwards, limiting how they can move. It seems like a small difference, but it can have a big impact on how you can design environments.
Let's consider this problem. The player needs to get to the X:

How can we design the map to make it more difficult to reach the X?
In a top-down game, we can put some walls in the way:

But we can do that in a sidescroller too:

We could put a locked door in the way. That way the player can only get to the X if they have the key:

But we can also do that in a sidescroller:

Almost anything you can do in a top-down game can be done in a sidescroller. The fact that a sidescroller has gravity is something extra which lets you create different types of obstacles.
Let's look at a different situation. Here's a different map where the player needs to get to the X:

In a top-down game this isn't a problem. The player just moves up, then left.

In a sidescroller we can create many different ways for the player to get to the X. We could make a ladder:

Or a moving platform:

Or a series of tricky jumps:

You could make a rope that the player can use to swing across a large gap. You can make a trampoline that bounces the player high into the air. You could make the player fly or climb walls. Or, maybe the player has no way to get to the X and needs to unlock new abilities first.
The map is always part of the game, but with the standard top-down view the map is often inconsequential. Imagine an RPG where the player walks into a room and sees the boss. The player approaches the boss and initiates the fight. The map shows you what the room looks like but it doesn't impact the gameplay. Whether the boss is on the left side of the room or the right, it's still the same to the player (approach boss, start fight). In a sidescroller the map is better integrated into the gameplay. You can create more interesting obstacles so the map is much more than just something pretty to look at.
I've often thought that making maps for BYOND games was one of the most boring tasks. You want to restrict access to an area? Put a locked door in the players way. It's boring. In an action-platform game the player has gravity to contend with. Your maps can create different, more elaborate types of obstacles.
If you feel inspired, try developing a sidescroller of your own!