The name of inlogic's game says it all - you have a house and you need to defend it by building different kinds of towers along the access path. Your house is a nice castle, in fact, and the attackers are trying to break it into pieces. If your defence isn't sufficient, they succeed, and your castle is gone...
Tower Defence follows the rules of the genre tower defence pretty well and if you've already played any you can start without instructions and you immediately know what to do. Well what to do is quite obvious. Kill everybody who jeopardizes your pretty castle with moat and red brick roof, and do it before they get to the drawbridge for once they arrive there, the situations gets serious.
The game suggests where you can place the towers and you choose what kind of tower you want (and can afford at the moment), and which place is the most strategically advantageous. You are paid for each killed monster and your towers can be upgraded and turned into more effective. If you change your mind, the tower can be sold. In urgent situations you may use magic and throw on your enemies fire ball or sprinkle them with toxic rain. Nice.
The game is completely controlled by a mouse.