Planters built into patio
Any advice regarding building a patio with built in planters at the edges (to act as a barrier to avoid folk stepping off the end). Patio height will be about 600mm from ground. Top of planters to be flush with patio height . Particularly interested in (1) how to approach drainage - eg would filling in half the planter with rubble be appropriate? And (2) materials to use for constructing the planters? Is simple blockwork likely to be sufficient or are there additional factors to consider? Thank you in advance for any thoughts you have.