A raiding party reaches a castle gateway. The garrison drops a heavy grille into the entrance passage and raises the crossing over the ditch behind it. Which two features have they used?

Explanation

The grille that slides down vertical grooves to seal a passage and the span that can be lifted to break the crossing of the ditch are two different fittings working together at the same entrance. Bringing in a battering ram fails because that was a tool carried up by attackers to break a gate open, not a device the defenders could operate from inside.