Dungeon Keeper's waiting times will make you gasp like someone's jammed an un-lubed fist up your ass. Four hours to upgrade a room. An entire day to dig through a single block. These are measurements of actual time. As the overseer of a fledgling dungeon, you must mine resources, use imps to upgrade, while expanding your underground fortress, summoning raiding parties and making essential upgrades. The obscene downtime is there for the sole purpose of forcing you to exchange gems for cash in order to perform the required action immediately.
Gems, the detestable lifeblood of this experience, can also be spent on tantalising (yet temporary) upgrades for your dungeon, alongside bulk resources and other useful items. They can be earned through deeds such as extensive mining, or raiding enemy dungeons. But the amount accrued is utterly insignificant. For example, unlocking early achievements (four hours in) will net you five gems - to purchase a third imp costs 800. It soon becomes apparent - it couldn't not, the way the game browbeats you with messages asking for money - that the only viable way to progress is to buy gems via micro-transactions. And, low and behold, they are not cheap.
Any positive steps the game may have made through its strategic attack and defend scenarios, cartoonish charm and decent sound effects are soon consumed by rising bitterness over its blatant ploy to siphon cash from your bank account. A key frustration is that it didn't have to be this way - if only the waiting times weren't so egregious and resources could be spent on gems the experience would be so much more palpable. As it stands, it's a terrible example of F2P.
This isn't Dungeon Keeper. It's a cloaked figure badly imitating an old friend, beckoning you into a tar-black alley. Under the shroud is greed incarnate, grinning.
Version Tested: Android. Played for 6 hours.