Lilia
Lilia (Latin plural, meaning "lilies" in English; singular, lilium) are pit traps arranged in a quincunx pattern dug by the Roman armies in front of their defences. Frequently they had sharpened stakes set inside them as an extra obstacle to attackers. 

Lilia have been found in front of both Hadrian's Wall in England and the Antonine Wall in Scotland.

External links 
Images of lilies at the Antonine Wall

Fortification (obstacles)