Burrow allows a creature to move through solid stone at one half its burrow speed. Does this also allow a creature to move through non-floor surfaces, such as walls? Could a creature with burrow travel through the city walls into a city?
Does burrowing through vertical surfaces allow vertical movement? Could a creature with burrow move into the city walls, then up to the allure behind the battlements?
If a creature can burrow up a vertical surface, can it also burrow through a stone ceiling? Could it then drop down on an enemy below, or travel up to the floor of the room above?
See this closed question for prior discussion on the topic of burrowing.