Pyramid will be unlocked when you find all the skystones and upgrade the City Gate to Level 2.
To restore the Pyramid find necessary collection items. Complete collections to get these items. To build the Pyramid you need resources. When the Pyramid is ready you will open new levels in Golden Tablets Mode and find Gold Runes.
It takes time to find rooms. You can speed it up.
There are 6 available levels of the Pyramid and 105 match-3 levels to beat. To upgrade the Pyramid you need to fulfill the requirements. You also need resources to upgrade the Pyramid. Upgrading takes time but you can speed it up using your crystals.