More or less everywhere.
If you just use it you are up/down right away. This is the most commonly found ladder.
In the Summer Update 2008 the image used for a wooden ladder was changed. The sprite was changed because many new tibia players found it hard to know where exactly to click to use a ladder and get up. Since the ladder stands in the middle of a square now, it's easier to see this.
It used to be like this: