This is all inside of the framed bed of coarse.

Something like this but it's all about filling the bottom

For my raised bed, shown in the photo, I used the layering technique. Bottom layer - (1/4 inch thick) Cardboard to prevent weed/grass growth and encourage earthworms. Layer 2 - (4 inches deep) Shreded leaf compost obtained from the local recycle center free. Layer 3 - (4 inches deep) Top soil purchased on sale from HD for $1.27 per bag. Layer 4 - (2 inches deep) Humus/manure mix puchased from HD on sale for $1.89 per bag. Top layer - (2 inches deep) Garden Soil purchased from HD on sale for $3.39 per bag. Growth of plants has been very good. I add humus and manure each year.

At 30 inches tall your not going to be really growing into that bottom 18", so you could definitely fill. I would be hesitant about pallets, a lot of them are treated with pesticides and whatnot, for their original use, which may leech into your soil. If you know your pallets are untreated, I don't see what it could hurt.

Most pallets are stamped. If the stamp says HT, that means they are heat treated and are safe to use.