Creating a world file with a series of cylinders as obstacles

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you might be looking fot the population.

It's a characteristic of the world. So you put the tag directly under the <world> tags. You can choose from different layouts. Depending on the layout you pass the tag sertain information, like number of object (in case of a line or some random layout) or number od columns and rows (in case of the grid). You can find all the information in the two provided links.

You want to spawn the cylinders after the world is loaded, or to have the cylinders in the world from the beginning?

It sounds like you could start from this tutorial.