But there are trenches in the movie.

Also, defending the trenches:
1) They're good cover.
2) They're meant for defense and counter what the Imps might throw at them. It doesn't matter if the Rebels have any vehicles or not.
3) Walls might attract to much attention (especially if they're made of unnatural materials (read: not snow or ice)). Trenches are somewhat smaller and harder to see and detect. Also, they don't stand out that much - from a distance, it's harder to see a hole than a wall. And don't forget: the Rebels came to Hoth to hide, regroup and resupply, not to fight. That's why when they were attacked by the Empire, their first thought was to run. The second - to fight and buy enough time to run.
4) If the Rebs had walls, they'll put them around the bases and critical instalations, not in a valley, where the Imps might pass... or not.

Of course, the final call is yours. You're designing and making the map, not me.

Does anyone know which object the t1-b is?