Nice Start. Here are some ideas to think about:
1) More Arachnid Types
2) Let the towers change with updates e.g. grow bigger, add Spikes
3) Give the Player Spells or something... make the Player feel more involed
4) Make a towerupgrade preview so you can see what changes stats wise
Those are my ideas just to keep you thinking.
otherwise what BowWow200 said.