Tech level? Medieval.

Magic? Yep. More specifically: using levitation.

Forests surround my town, which is built in a wild forest on a river trade route to protect merchants. 

**How would I build my town to be spider proof?**

The spiders are five feet tall and three feet wide, with web spinning abilities and poisonous venom.

For the sake of this question: please ignore any reasons a giant spider cannot scientifically exist.