The core gameplay of Skytear Horde is the combat between waves of monsters and your hero and troops defending the castle. If you played card battlers like Heartstone or KeyForge, you will find yourself at home.
You lose if your castle is destroyed or if your deck runs out of cards. On the other hand, to win it will not be enough to "just survive" the horde. You will have to find the resources to leave the castle and tear down the gates that are spawning the waves of monsters.