The Horlek Fortress is an abandoned fortress located at the ruins of the east side of the Mushroom Kingdom. It's more difficult than Goomba Valley, having more and stronger enemies. In this location Mario and Luigi learn how to use the Hammers out-of battle and in-battle in Mario & Luigi: Rivals Quest.
The main enemies here are Bob-Ombs and Bullet Bills, and also, rarer and stronger enemies such as Chuckyas and Chain Chomps. There are 3 bosses, Bowser (first time), Big Bob-Omb and Lord Ztar (first time). Horlek Fortress connects to Goomba Valley and Leafiholly Woods via door and to Toad Town via Warp Pipe. There's also a hidden Whacka.