Zombs.io is an interesting 2D tower defense IO game freely available on the Internet. Recently, an iOS edition was published, so smartphone and tablet users with those operative systems can download it from the iOS Store.
In this game, the main task is to collect as many points as you can and to reach the final leaderboard's row. There are a number of available modes to choose from. You can play it with your friends or in a solo variant. That, of course, depends on your wishes. Zombs.io is a popular international game, so you can expect to share the platform with other competitors from different parts of the world.
Your small yellow character is supposed to collect nearby resources and to build a base using them. When you finish with the building, you will be tasked to defend the base from aggressive zombies that attack at night. You have an option to buy and upgrade armor and different tools that will support your mission.