This set him planning how the space might be used for building.

In the angle, the rock came above ground entirely, and had been made the foundation of a wall connecting the two corners, to defend the court--a thick strong wall of huge stones, that seemed as solid as the rock.

He grew fond of the spot, almost forsaking for it his formerly favoured stone, and in the pauses of his gardening would sit with his back against this wall, dreaming of the days to come.
Here also he would bring his book, and read or write for hours, sometimes drawing plans of the changes and additions he would make, of the passages and galleries that might be contrived to connect the various portions of the house, and of the restoration of old defences.

The whole thing was about as visionary as his dream of Tree-top-city, but it exercised his constructive faculty, and exercise is growth, and growth in any direction, if the heart be true, is growth in all directions.
The days glided by.

The fervid Summer slid away round the shoulder of the world, and made room for her dignified matron sister; my lady Autumn swept her frayed and discoloured train out of the great hall-door of the world, and old brother Winter, who so assiduously waits upon the house, and cleans its innermost recesses, was creeping around it, biding his time, but eager to get to his work.
The day drew near when Cosmo must leave the house of his fathers, the walls that framed almost all his fancies, the home where it was his unchanging dream to spend his life, until he went to his mother in heaven.
I will not follow his intellectual development.