The Portcullis was a super-heavy tracked vehicle once used as a mobile energy shield generator by the Legiones Astartes during the Great Crusade and Horus Heresy eras of the late 30th and early 31st Millennium. The Portcullis was outfitted with extremely powerful energy shield generators, similar to starship Void Shields.
The Portcullis tanks of the Dark Angels Legion featured an even more advanced design that was known to and only used by the I Legion. These vehicles' shield generators were used to protect allied infantry and armour from enemy ranged fire.
A portcullis is a strong, heavy grating that slides up and down upon vertical grooves and was used during medieval times to block a gateway to a fortress or town.
- Angels of Caliban (Novel) by Gav Thorpe, Ch. 16