Match-three gameplay, of the drag-a-line kind rather than the swapping kind. As a match-three game, it's excellent. Different colours charge up special moves, such as shuffling the board, or breaking frozen tiles, etc. Often you need to plan ahead and think about your chains will affect the distribution of the board, in order to eventually target a certain tile.

The main objective of the game is city-building, but that aspect is shallow and too linear. I wish it was more fleshed out, and affected the gameplay somehow. I thought the buildings I was unlocking would give me special powers in the tile-matching parts, but it was just window dressing. Wasted potential there, but it wasn't annoying.
One thing I was impressed with was that your god's-eye view of the city allowed you to see dozens of citizens milling around, performing various tasks, and each one of them was named and can be clicked on to read their thoughts.

Occasionally there are object-hunting minigames, which unlock the special moves I mentioned above. This was the only part of the game that I actually disliked. I am very into hidden-object games, and there's a right way and a wrong way to do them; and it was all wrong here. The way the objects are hidden is obnoxious, forcing you to tediously hover over the entire screen to see the mouse cursor highlights.

But most of the time, you'll be tile-matching, and it's really good. It's got depth, with many mechanics are that introduced bit by bit. I mean, I spent 60 hours on it, and that was just the campaign. There's a bunch of challenge levels afterwards.