Stone retaining walls are great for growing plants between the gaps within the wall.
Old walls with crumbling mortar and plenty of holes are ideal to grow alpines as well as a large number of other plants in the gaps.
Newer walls are not as ideal, but if bricks or stones are removed here and there then you can use those gaps for planting.
One way of getting plants to grow in holes is to mix seeds into balls of soil and fill that into the available gaps.
To select the best plants for your wall garden, first you need to know how much sun and rain your wall gets.
• Plants for shady walls:
o Asplenium and Cetarach species (ferns)
o Haberlea rhodopensis
o Ramonda myconi
• Plants for sunny walls
o Alyssum saxatile
o Armeria species
o Erinus alpines
o Erysimum alpinum
o Gypsophila repens
o Linaria alpina
o Saponaria ocymoides
o Saxifrage species
o Sedum species
o Sempervivum species
• Plants for very sunny, dry walls
o Cheirantus varieties
o Dianthus deltoids and other small, cushion-forming dianthus
o Helianthemum serpyllifolium
o Lewisia cotyledon hybrids
o Phlox subulata
These are only some possible examples of what you can use in your wall garden. Would be great to hear from you if you had a go at creating a wall garden.