Can you run up and down ladders?

For a long time we have been playing that ladders are difficult terrain and therefore you cannot run while on a ladder.
We recently had a new player that questioned this rule and we realised that it doesn’t say anything in the rule book about ladders.

I was wondering what rules other people used?

Printed Mordheim rulebook p. 27, re-read the paragraph titled OPEN GROUND. Ladders are considered open ground in the main rules.

Ah thanks.
I must remember to pay attention to basic rules rather than just looking at shiny new warbands and settings.

But being at the top of a ladder still counts as "Defended Obstacle" for being charged, right?

If you house rule it it does. There really isn't a defended obstacle rule in Mordheim, although one appears in the scenario Encampment Raid in one of the last TCs. We used to have such a house rule, but it got applied rarely because the group wasn't really into it.