To hold off said monster hordes, you can build various offensive and defensive modules in the rooms that you've discovered, tower-defense style. If you have the resources, that is. There are four resource types: industry to build modules, science to discover new module types, food to heal your characters or recruit new ones, and dust to extend the power of your energy crystal to adjacent rooms. The dust resource is particularly useful, since extending crystal power to a room means no monsters can spawn in that room.
The graphic style is pixel-art, and the user interface is fairly minimal. The game suffers a bit from this combination, as it becomes difficult at times to tell what exactly is going on. It's hard to tell the difference between module or monster types when the screen gets crowded. There's no tooltips to tell you what modules are in a room, or notification when one is destroyed. Not a problem in the early floors, but later on when things are spread out across a wide area, it can be tough to keep on top of everything that's going on.
Dungeon of the Endless is almost entirely about making the best use of limited resources. You'll almost never have enough dust to power all the rooms of a floor, enough industry to build all the modules you'd like, etc. Some floor layouts will allow you to build up a defensive gauntlet of modules that monsters must come through to get to your crystal, but others are spread out wide so the monster hordes must be faced from all sides. Powering up the right rooms is an especially important tactic, since it lets you limit the areas where monster spawns can occur. If you happen to get a wide-open floor layout without much dust to power up rooms...well, there's always the next escape attempt.
When you start the game, you're given the choice of "Very Easy" or "Easy" mode. These descriptions are complete lies. "Easy" is no such thing, and "Very Easy" can be more accurately described as "Gives you a chance, if the random floor generator doesn't decide to give you an impossible layout." I played maybe a dozen games on the default "Easy" setting before I realized just how difficult things were going to be, and switched to "Very Easy" mode. Presumably there are more difficult modes unlocked if you manage to win on "Easy", but I don't think I'd ever want to put myself through that. Probably there are better players out there both talented and masochistic enough to do so, but I certainly am not one of them.
Each attempted escape run in Dungeon of the Endless has the potential to unlock more characters and achievements. If you come across a character during your travels, recruit them, and keep them alive for a few floors, then they become unlocked to use as a starting character in future attempts. Achievements take a couple of different forms: "stories" where a particular combination of characters survive a few floors together, and "pictures" that unlock as you accomplish certain goals. (Fight off a certain number of monsters, use a particular set of modules, etc.) In between escape attempts, you can view the unlocked pictures from the main menu.
Each character has a bit of background and some special abilities, most of which are amusingly written. Each character also spouts off occasional one-liners while going about their business, and the group will have conversations during elevator rides from the exit of one floor to the start of another. These are usually entertaining, and sometimes part of the unlockable stories. I wouldn't play Dungeon of the Endless just for these bits of flavor, but it certainly adds to the enjoyment along the way.
Dungeon of the Endless can be frustrating at times, but nonetheless has been a fun experience for me. I enjoyed the discovery aspect of meeting the different characters and unlocking some of the pictures and stories. It's worth picking up for anyone that enjoys tower defense-style games and would like to try one that works a bit differently than most.