Wingnut
Footballguy
They waited a bit until the NK commanded them to do it ..and who cares if they lose a ton of troops, he can just make them rise again at will. The NKs end game was to kill Bran and create endless night, and as long as the dead army got him inside so he could get to Bran, that's all he really needed them for.Isn't that kinda what they did? But just as a bridge - rather than smother the whole thing? Which would have been wasteful of troops.
The way they smothered the Dothraki in seconds, it seems like an 8 feet wide fire moat wouldn't have slowed them down at all. But I've never commanded an army of dead, so what do I know?