In the best levels, the level design structure makes you feel like you are exploring areas you are not supposed to, while never being stopped in a dead end.
Most levels in Spyro the Dragon (and the other games of the trilogy) use (loosely) hidden treasures and secret paths to push for exploration around a simple critical path. Some are cleverly structured to guide the player’s exploration all the way through 100%. Here is a case study of a few of them.
This is the level where you are taught to glide. The critical path (in white) alternates between situations with the bull enemy and jumps that require to glide, each with increasing difficulty.
The second part of the level (in yellow) loops around the first part. It is always accessible from a visible platform (on the left of the screen @2:08, in the video walkthrough below), but a new player most likely will not see or be able to make that jump. Going through the whole bonus loop gets you to a high area from which both the end and the start of the level are accessible.
This level starts with a linear progression. The two parts of town are a bit open, but split by a single bridge. Treasures on top of the buildings are teased but not yet accessible. Then, climbing to the highest part of town (starting @2:35) is also perfectly linear.
Once at the upper point, you find the exit and a dragon that tells you that you can glide to another part of the level (@4:07). The obstacles (buildings) of the town can now be used as platforms to access the treasures. But a completely new part of the level is also accessible to explore, through a long glide that has to start next to the exit point (@5:00).
This level embraces the open areas it provides (the first enclosed area, the second one on the right of the map, and the whole upper part), and the critical path is very short.
There are multiple rooms to explore but most are dead-ends, which is not what I’m interested in here. The most important one is the tower in the center of the right area, from which you can glide and get access to what appeared to be inaccessible landscape (jump @4:40). Even more than with the previous example, you are given a higher point of view and freer access to the level.
What I’m highlighting here is not true for all of Spyro’s levels. I’m only taking as references the levels that manage to create that exploration feeling while still guiding the player.
What those levels have in common is a basic part to teach new mechanics / enemies, a more open part with an obvious critical path plus some bonus areas to explore, and finally a full bonus part that is either hard to access or accessible only after the critical path is done.
The first goal, finding the exit, is achieved at the end of the critical path, but the game encourages you to explore to reach the real goal of 100% completion.
That level design makes sure the player is not lost with a clear path to the exit, but gives many opportunities to explore more. You are encouraged to find everything through gameplay rules (the completion rate is displayed in the menu and the progression is locked until you’ve found a certain amount of collectibles), but the levels are made so you want to explore them.
The best bonus paths also avoid putting the player in a dead end. The level’s exit itself is rarely presented as a dead end. The fact that the majority of the level’s content could be skipped makes it feel more like bonus or secret areas, even though it is the core experience. The feeling of going where you’re not supposed to is increased by sometimes disguising the platforms as decor (columns, house roofs…).