More than a fence-- if someone wants in, they would be able to get in easily.

And that's not mentioned yet how people would be able to gain access to the roof in the first place. [he pauses in his writing, twirling his pen between his fingers.] The answer to that is "painfully easy".

Someone, or some group of people, leave planks everywhere to cross from one building to another. The poor greenhouse would just have people thundering past it all day.