With Luna away, Celestia set about her task, which was to do some housekeeping. She was to build the Royal Library from selected materials that had been a part of their library in the floating alicorn fortress. Pony civilization now numbered three hundred and ninety, as many foals had been born over the past four years.
It would be some time before ponykind needed to expand beyond Ponyville and explore the ruins of the Old World. Still, Celestia's duties were to plan for millennia into the future. If ponykind was to once again cover the globe, they needed education and infrastructure. The Royal Library would be a good place to start.
Celestia kept the books containing the most dangerous magical theory locked away safely in the fortress and divided the rest between commonly useful texts and advanced texts for trusted eyes only. It would be important to eventually build a sect of talented unicorns who would serve royal functions for the good of ponykind, using this advanced knowledge.
It would still be centuries away, but Celestia was already planning the establishment of a Royal Palace, a Royal Guard, and a School for Gifted Unicorns. Meanwhile, Celestia herself would take on an important teaching role, as long as there was peace in the land, and raise up educated ponies who would spread her vast knowledge to successive generations.
One order of business was to hide the fortress. It was massive—the size of a palace itself. Hyperion had created it to be powered independently, so it could fly even in his absence. Celestia chose to bury the fortress beneath the ground in the Wild Lands, far from any populated region, and constructed a common looking settlement above it on the surface.
This served as a place where she and Luna could visit to talk and think, away from other ponies. It was remote enough where it would be quite some time before curious eyes discovered its location. And when they did, they would find nothing special apart from some alicorn's trinkets.
Celestia began a journal of events surrounding her parents and the reason for the fall of ponykind. She documented the facts behind alicorn metaphysics and how a fully matured alicorn might be killed. The purpose of the journal was to document how an alicorn can be defeated in case for some reason she was no longer alive when the need arose, and as a backup plan in case Luna's weapon failed.
She also used the journal as an outlet for her personal feelings regarding the current era, the rebuilding of pony civilization, her thoughts as ruler, and observations on mortal pony nature. Celestia kept the journal in a locked chest in the basement of her writing studio, which was built above the buried fortress.