Just google other examples and study how they created depth. e.g.
- Zelda 1 uses radial lines (not ideal for freeform dungeons)
- Zelda 3 uses a tonal gradient (i.e. walls get darker towards floor)
A hypothetical solution would be to reduce contrast toward the top of the wall (i.e. don't use the darkest or lightest tones of your ramp, and blend the areas where harsh light meets harsh dark). This will (in theory) make it look out of focus, giving you a depth-of-field effect.
But as others have noted, the bigger issue here is that a ) the floor is too similar to the walls (hard to see at a glance where the wall stops and the floor starts), and b ) the floor is too high-contrast. Since all your busy action (sprites, blocks, etc.) is happening on top of your floors you can't make the floors themselves too busy or it's overpowering. If you compare LttPs floor colours to the wall colours you'll see they're all much more similar in tone (i.e. lower contrast). This makes the walls stand out against the floor and makes the scene more readable. They're also a different colour which helps a lot too, but isn't strictly necessary.
If you're working in the classic set with a limited palette and you can't make the floor colours lower contrast, make the floor texture lower contrast. Use lots of flat colours with suggested details (e.g. Minish Cap) rather than lots of small tiles. Even Z1 had big, flat floor tiles in order to keep your eyes away from it. Because remember, the point of floor is to not be distracting. Keep your visual points of interest to walls, water, objects, sprites etc.