What does the animal feed do? I've seen a chicken eat it and a heart appears over its head briefly, but aside from that does it have a use?

When you build a market you can click on it with a tool to open the purchase menu, or you can click on it with an item and you will throw that item at the market and it will give you coins.

Smack them with your hammer

Just decorative in the demo. In the final version they will make you walk faster and probably reduce energy consumption as well!

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You have to build the market and to sell you simply equip the item you wanna sell and you throw it at the market.

You make crops with the Shovel which you unlock pretty early in the game, I'm not sure when you get the windmill but you get it after a certain level I think :)

Acelerar predios proximos

I know i'm VERY late, but you throw gems on them, that you get from mining.

You unlock the Windmill when you create the Shovel. I've only found 3 things in the Windmill build menu (Animal Feed, Wheat Seeds, and Tree Saplings) and they were there from creation of the Windmill. 

You can destroy every structure (except the Marketplace, unless I haven't figured it out) by smacking it with your pickaxe. The structures that build things must be in the process of building something and then you can smack it til it breaks before it finishes the build. You receive all of the materials it took to create the structure in the first place, so don't worry about losing the materials. However, I have discovered that if you destroy a Furnace while it is making something using Coal, the Coal will not be returned. Lost 64 coal canceling Gold Ingots :(