Hello.......I am new to the site, so bear with me. Need some advice on building a raised planting bed "directly on top of a blacktopped surface". A job I am bidding on is required to have 20% of their property "green space"- per village ordinance (business). They need (2) raised planting beds, with misc shrubs/perennials in the front of their business........on top of the parking lot. they are not required to tear out the blacktop. I was thinking layer of gravel, with some drainage piping (perforated) before dirt added, and placing landscaping blocks around.........besides drainage issue and soil runoff, I think my biggest problem will be making the wall level....due to the uneven parking lot, and not being able to "sink" that first stone in the ground. The wall (bed) w will be (3) blocks high with a capper on top. Any suggestions?