Like it says on the tin, you're a hitman where the object of the game is to kill your target. What makes this great is the immersive free reign you have in each of the levels, despite it all being scripted. (Replaying a level is a lot like Groundhog's Day, the same events happen over and over.) Heavy emphasis on steath-based gameplay, don't go guns-ablazing.

For example, while it may be easy to kill the opera singer while he's taking a break, it's much more rewarding to replace the prop gun with a real gun and waltz out like it's not even your fault.

I'd put the base game at 10-15 hours, but if you want to actually figure out the "real solution" instead of looking it up on GameFAQs, you can spend some time just monitoring the guard's patterns and just searching the area.

I put probably too many hours in this back when it first came out. In short: you are an Evil Genius who is building a secret lair to do nafarious deeds. You, the Evil Genius, don't do those deeds but send your minions to do it for you. It's kinda a Simcity in building out your evil lair to train henchmen, but also to keep do-gooders out with all kinds of wacky traps. Use cameras, auto-turrents, gas traps, buzzsaws, and/or the hotel front's booze to keep secret agents in check.

Pro-Tip: There's an official 1.01 patch somewhere out there you can apply to the steam directory (dunno why the Steam version is 1.00) and an unofficial balance patch. Check the Google if you really like this and want the couple of wee fixes.

While pretty much a standard fare tower defense, it's well-polished, balanced, and super fun. Should be an insta-buy for the full game + all DLC on it's daily deal during a Steam sale. For $4, it will amuse quite the weekend.

Note: Play the campaign first. You may be tempted to go direct to the Portal DLC levels, but the "You Monster" levels are actually quite challenging.