I Only Summon 1-Star Heroes

Logan Winters, a world-ranked hardcore gamer, is suddenly trapped inside The Last Bastion, a brutal Tower Defense RPG. Inheriting the decaying fortress of Evergarden, he must defend it against escalating waves of monsters. To build his army, Logan must use the game's Gacha system; however, for some reason he can only pull 1-star units. Using his game's knowledge and exploits, Logan sets out to rebuild his fortress, survive the horde, and uncover the truth behind his broken luck. — Description from novelcodex.org