In addition to the story mode, there is a never ending survival mode. It's never ending until the enemy reach the trophy that it is. There is a set map for survival mode and you can play to your hearts content. As the difficulty ramps up, it may be worth selling and demolishing some of the cheaper towers for the more powerful ones.
Bionicle Defenders also has barricades and special tiles that can be placed on the path. There are several types of towers such as ice and there are some enemies that are weaker against ice than others. However, you can lay an ice tile and then everybody can feel the icy pain. The tiles add another layer of tactics and your defence has to be well structured.
If you follow a few basic rules then you should make good progress and make the most out of this very niche game.