I've played quite a few flash games that involved hotel management, and Unholy Heights does a good job of incorporating new mechanics to a classic genre.
If you like hotel simulators, monster themes, defense-based gameplay, and fairly casual pacing, then you will enjoy this game.
The aesthetics are nice - the detail in the characters is decent, and any upgrades you purchase for each room is never shown. The music and sounds are okay, nothing too special, although I personally like to mute the sounds in favor of my own.
The tutorial is quick and very helpful at showing you how every mechanic works. The learning curve is minimal to moderate, and it is easy to understand everything about the game with one or two short playthroughs. The pacing for the game is completely up to the player, as you can continue grinding until you wish to increase the difficulty (by purchasing the next floor for your apartment).
In the game, you play as the Devil, acting landlord of the Unholy Heights apartments. You can accept different monsters as tenants, and these units are called upon whenever you are attacked by adventurers. The monster types you can add as tenants increases as you progress through the game and build more floors (this also increases the overall difficulty).
Each monster has different desires, and purchasing their requests (room upgrades, like better computers, fridges, etc) will result in a satisfaction boost. This is useful because you can then increase the rent a little without incurring a penalty. At high levels of satisfaction, monsters will have bonuses to their abilities.
During battles, you can knock on doors to ask tenants to fight for you (this is an interesting mechanic, because monsters at work will be unavailable, however, monsters that are unemployed are also less likely to pay rent on time). Your monsters will fight in a line, so clicking on a monster who is about to die will allow them to retreat (and bring the next monster in line to the front). You can purchase one-time consumables (like health potions) that are used during battle and when a monster returns to his/her room. If you fail to kill an adventurer, he/she will take a percentage of your money - and escape with it if you are unable to kill them by the time they exit the screen. Saving often is encouraged, since you can have your entire tenant list destroyed if you make a mistake and/or do not plan accordingly.
Monsters can also meet mates (usually if they are employed and have better rooms), adding another person to your defense cue. Eventually, the couple can reproduce, earning you a third monster for a single room. Being able to keep families alive and happy is a great way to increase both your revenue and defense rating.
By default, adventures will appear fairly infrequently, however, you can access missions for gold rewards - a difficulty rating and brief description gives you an idea of what to expect. In order to build the next floor for your apartment, you must defeat a boss mission and have enough gold (quite expensive, so grinding is recommended).
Overall, the game is a nice, casual time filler. If you are interested in hotel management and/or defense games, definitely give this game a try (or at least take a look at the demo).
DJSF @DJSF's Rogue Reviews