Wall

This tower can be placed on the track although it dosnt deal any damage (without upgrades) it has a decent amount of health, although any creep can attack it, even ones that dont usually attack, although they deal relatively low damage (dependant on level)

Base stats
Health: 1000     Cost: 250

It looks like a Generic wooden block

Level 2
Health: 2000     Cost: 500

It looks like a concrete block

Level 3
Health: 3500     Cost: 750    Damage: 50

It looks like a steel block with spikes on the front allowing it to deal damage upon contact

Maxed level
Health: 5000     Cost: 1000    Damage: 100

it looks like plate steel with sharper spikes