City Bloxx.
The idea is to build a city in a 5x5 square.
You have 4 types of buildings of increasing height; blue,red,green & yellow. In order, each requires the previous buildings next to it ... so blue can be put anywhere, but red buildings have to be next to blue, and yellow requires blue,red & green in adjacent squares. You can build over old buildings.
Building height is a factor for population (score), but I spend more time wondering how to get as close as possible to all yellow buildings.
(yeah, I know it's not much compared to any real pc games ... it's just something to pass time while I'm on the train)