Structures are special objects that serve a passive purpose such as using a wall to defend or blockade an area, or an active purpose, such as a Houndius Shootius being used to protect an area against mobs. They are either crafted in-game, or sometimes found around the world.
When placed, every structure or plantable has a radius. Nothing new can be placed inside of this radius unless the structure is destroyed. It doesn't take into account what is already there; it only counts the radius of the object being placed. (This is why, for example, you can place a chest very close to an existing birdcage, but if you try to place a birdcage next to an existing chest, you need to place it significantly further away). The following are the radius values from the LUA files:
|Houndius Shootius||0.00||Can be placed anywhere on valid ground|
|Walls||0.10||Distance from other walls|
|Walls||1.00||Distance from things that aren't walls|
|All Other Plantables||2.00||Things that can be items|
|All Other Structures||3.20||Things that can't be items|
Note on Plantables vs Structures: For the purposes of the table above, if you can build it and it goes into your inventory, it's a plantable. If you can pre-build it and place it later, it's a structure.
If looking to build stuff tightly together, you need to place the large items first. (For example, build the birdcage first, then the walls around it, not the other way around. Build the crockpot, then the chests and so forth.)