This is essentially a turn-based, tactical dungeon crawl. The first time you enter battle, make sure you drag your team members to appropriate positions. Warriors and other short-range fighters should go in the row closest to the enemy, rangers and spell-casters in the rear. When a character's turn comes around, his portrait will be highlighted, and you can simply click on a creature within range to attack it. You can use special abilities (healing spells, special attacks, and the like) from the grid on the left by clicking on the ability and then on its target. Those who played the recently-reviewed Brute Wars will recognize the basic concept, which was probably inspired by the first Monster's Den.
There are three unique quests: the original Den of Corruption quest from the first edition of the game, now playable with the updated interface; the new Den of Terror quest, which features an extra boss fight on every level and a selection of rare but intriguing random events; and a survival mode where you get to massively power up your party and fend off endless waves of attackers. You have a choice between three difficulty levels and a number of other options to fine-tune your playing experience. "Hardcore" mode, where your game just plain ends when your party dies, is where it's at.