Sloped terrain is hard to do in a convincing way in an orthogonal 2d tilemap. But here is an example from the 1993 SNES game Illusion of Gaia by Enix which does a quite decent attempt:
This ramp uses 3 tiles:
What techniques are at use here?
- The diagonal cliff tiles provide a perspective hint
- The floor texture is darker than the one used for level ground. This creates a 3d illusion through shading. You could also use a brighter texture instead on the ramp in the opposite direction.
- Not visible on this screenshot, but the game mechanics also sell the ramp: When the player moves horizontally while on the ramp, it gets interpreted as diagonal movement. You can also increase the movement speed when moving downhill and decrease it when moving uphill.
Here is a more sophisticated example from a game with more technologically advanced pixel art. The original 1998 Starcraft by Blizzard:
This slope is more detailed and it isn't tileable. But in the end it uses the exact same techniques: Diagonal rock formations and the shading of the floor texture (in this case lighter) create an illusion of perspective.
And by the way, you should not steal these tiles. Both companies still exist.