They had come, with the creepers, into possession of the summer-house.

Consequently, the creepers were a part of the constitution of the summer-house.

And consequently the Owls were the guardians of the Constitution.

There are some human owls who reason as they did, and who are, in this respect--as also in respect of snatching smaller birds off their roosts--wonderfully like them.
The constitution of the summer-house had lasted until the spring of the year eighteen hundred and sixty-eight, when the unhallowed footsteps of innovation passed that way; and the venerable privileges of the Owls were assailed, for the first time, from the world outside.
Two featherless beings appeared, uninvited, at the door of the summer-house, surveyed the constitutional creepers, and said, "These must come down"-- looked around at the horrid light of noonday, and said, "That must come in"-- went away, thereupon, and were heard, in the distance, agreeing together, "To-morrow it shall be done." And the Owls said, "Have we honored the summer-house by occupying it all these years--and is the horrid light of noonday to be let in on us at last?
My lords and gentlemen, the Constitution is destroyed!" They passed a resolution to that effect, as is the manner of their kind.
And then they shut their eyes again, and felt that they had done their duty.
The same night, on their way to the fields, they observed with dismay a light in one of the windows of the house.

What did the light mean?
It meant, in the first place, that the lawsuit was over at last.