You must discover Lost Arts in order to gain access new types of Buildings and Units. Go to the House of Scrolls to begin discovering new Lost Arts. Upgrading the House of Scrolls will allow you to discover and upgrade new Lost Arts faster. Discovering each Lost Art requires Gold, Iron and Food. The more advanced the Lost Art, the more it costs. You may use Sapphires and Boosts to speed up the discovery.
The cost of upgrading a given Lost Art will become progressively more expensive with each Mastery Level. Lost Arts that unlock new Units have 20 Levels of Mastery, meaning that they can be upgraded to level 20. The higher the Level of Mastery you gain in a Lost Art, the stronger it will make the Units associated with it. For example, to increase the statistics of your Pikemen, you must upgrade the Imperial Steel Lost Art.
The cost of upgrading a given Lost Art will become progressively more expensive with each Mastery Level. Lost Arts that unlock new Units have 20 Levels of Mastery, meaning that they can be upgraded to level 20. The higher the Level of Mastery you gain in a Lost Art, the stronger it will make the Units associated with it. For example, to increase the statistics of your Pikemen, you must upgrade the Imperial Steel Lost Art.