The physics of soil hydrology are radically different between containers and gardens or raised beds. Raised beds have a never-ending wick to pull water from soils, or water flows laterally with the help of gravity to keep the soil in a permanently unsaturated state. In containers, downward water flow stops when it hits the container bottom and when soils are comprised of fine particles with tiny air spaces, water perches in a column at the bottom of the container or above coarse drainage material. (More on that in the link below.) Soils don't technically fatigue in containers like they do in beds or gardens. If we do not replace organic matter in our garden soils, they fatigue & become less favorable culturally. In containers, we grow in a near all-organic mixes (most often). They don't fatigue like mineral soils, but they do fail structurally. Large particles become smaller with age. They hold more water and less air. It's possible that a soil can be fine for a mature planting with roots fully colonizing the container, but a death sentence for anything over-potted. You should choose a container soil that will retain air for the expected life of the planting. When it stops retaining air, many will tell you that they rejuvenate it with the addition of "fresh ingredients". There may be some temporary truth to this, but imagine this: Compare a collapsed soil to pudding (made with water). They both hold too much water and too little air for plants to grow with vitality. Can we amend the pudding with an ingredient, the primary function of which is to increase aeration (perlite) to make the pudding a desirable medium? Why then, would we use things like collapsed soils or compost, amended with perlite & expect good results? So - soil fatigue in in situ soils can be corrected with the addition of organic matter. Soil collapse in containers is a structural issue & needs addressing via a different avenue. The best way is to replace the soil with a mix that guarantees good aeration and insures structural stability. I would turn the soil in your barrels into the raised beds fairly regularly, where they will be a welcome & wonderful addition, & start with a fresh soil in the barrels. Al Here is a link that might be useful: More info

In this case you would make the rows run east/west and then you could have enough room for everything you want.To make the most of your space, you could trellis the tomato plants using string veggie trellis like this http://www.homedepot.com/p/Unbranded-String-Trellis-928/202819857 along one outside edge of the garden and plan on 4 or 5 tomato plants on that 1ftx8ft row. If you use a heavy producing hybrid variety of tomato like Super Sweet 100 and larger variety if you like, you will have plenty of tomatoes. I would also tuck a marigold plant between each of the tomato plants to help with pest problems. My tomatoes always grow better with marigolds around. Plant a 2ft x 8ft row of eggplant, again about 4 or 5 plants, right in front of the tomatoes. Since you'll be able to reach the tomatoes from the outside, you won't need space between the tomatoes and eggplant. Make a 2ft x8ft foot path next to give you good room to move through the garden, then plant a row of pepper plants 1ftx8ft. Depending on the variety, you may be able to get 1 per square foot of this row, so up to 8 plants total. This brings you up to 1ft tomatoes, 2 ft eggplants, 2ft walkway, 1ft peppers with 2ft remaining for squash. Pick a compact variety of squash and plant them as close as possible to the outer southern edge of the garden with the intention of them hanging halfway over the outer edge. Make sure you've amended the soil with a good amount of compost to keep things growing strong. That's what I would do to maximize the space, but I'm no expert. There are also different schools of thought about gardening so do a bit of research and see what fits you best. I loosely follow the square foot gardening method with modifications I've adapted through trial and error. This picture shows my old garden from 2yrs ago using a trellis netting. I used that set up to plant tomatoes the whole 16ft length and in the picture I used it for beans, pumpkin and watermelon. That bed it 2ftx16ft.

Raised beds have the advantage of being in/on the ground, providing better drainage with no perched water. They also provide more space, more moisture retention, and take up WAY less space in ragards to how much you can plant per square foot. The advantages with growing in containers are that you can move them as needed, for storms, frost etc. Also, you can apply your fertilizer specifically as needed for each plant. Not to mention you can grow veggies in places where you cannot make a raised bed, like apartment patios and balconies. If I had to choose though, I'd definately go for the raised bed if I had the ability to do so. Joe

Further thoughts: I'd forgotten that dimensional lumber isn't actually the stated size. So 2x6s are 1.5x5.5 actual, and 2x8s are 1.5x7.25. If I did three board levels, the true heights would be only 16.5" and 21.75", respectively. And that's just the frame: in none of the raised beds I saw was the soil within 2" of the top. So, if I used three 2x8s, that would realistically mean a soil depth of about 19" or less. I've also sort of decided to make it 10x10 (or, two 5x10s) simply because I'll have trouble transporting 2x12s in my car. Ten feet is still tricky but more do-able, even if I'll have to buy/transport in batches. Plus, I was thinking about making an EMT conduit trellis for the tomatoes. Since that's sold in 10' lengths, that would make it almost perfect to affix to the sides with clamps (it would be going onto rebar buried in the ground, but the clamping would be for added stability). As far as compost goes, does it really need to be 50/50 for the whole depth? I.e., if I'm doing about 18-19", do I do a compost mix all the way down, or just the top half or two-thirds? On a similar note, I have a pile of sand around already. Should I spread that down for the bottom few inches, to aid in drainage? Flooding would be coastal--i.e., salt water--meaning that my only chance is to get the beds high enough to avoid it altogether, which is why I'm aiming high. Any really major flooding and I'm screwed (but in such a case I'll have way more to worry about than the vegetable garden).