Making certain units immune to chance-to-miss on heights?

I have a little detail on my map that bothers me more than it should, but still.

The magic towers on the screenshot actually have range til the middle of the bridge where the enemy is the most vulnerable. But it seems like the bridge is considered higher terrain (although it's not), and makes the towers 'miss' a lot. Well, they shouldn't.

Is there any way at all to fix this? Either by making the towers immune to the height effect, or rather (and preferably do something to the terrain)?

I guess the answer here is simply "live with it, increase the damage instead", but I've gotten so many good answers here at these boards that I'd thought I give it a shot..

Raise the terrain slightly where the towers are? (A full cliff level). But then melee units can't reach it and ranged units might miss. Hmm...

Raise the terrain for the towers, and make them invulnerable. Then make some kind of "generator" for them (see Campaign Map: Human X5). When the generator dies, so does the tower.