In Clash of Clans, your objective is to build a base and prepare defenses against incoming attacks. At the same time, you can train troops to use in battles against other players or AI bases.
When defending your base, preparing the right layers of defenses takes a lot of thoughtful planning. Choosing what to place and where to place it can make a huge difference on the outcome of a battle. For example, placing a cannon strategically can be used to take down any attackers before they reach your town hall.
Attacking offers the same level of strategic planning. You must choose carefully where to place troops on the map. When attacking other bases, you must destroy a certain number of buildings to be given a victory. The more buildings you destroy, the better rewards you'll be given. Destroying some buildings, such as the town hall, is required for victory.
The flow of gameplay is ultimately determined by the time-based building mechanics. It takes resources, and most importantly time, to train new troops and build new buildings. Alternatively, you can spend real money on the game, to speed up the building and training times. Players that don't want to spend money on the game are forced to wait, especially in the later stages of the game, sometimes for hours at a time.
This makes Clash of Clans a great game for those able to spare a few minutes each day - they can prepare their day's building plans and training of new troops, and then come back the next day, when new troops and buildings have been created. This also makes it not such a great deal for those looking for a game to play for extended periods of time at once. The game can sometimes be a little restrictive for those unwilling to spend real money.