The language spoken by the Woads is a combination of ancient Gaelic and ancient Welsh. The actual language spoken by the Picts has been lost.
Ray Winstone spotted Ioan Gruffudd curling his eyelashes one morning before shooting. Winstone spread the word, and Gruffudd earned the nickname "Sir Lashalot" for the duration of the shoot.
Stellan Skarsgård turned the role of Cerdic down three times before director Antoine Fuqua was able to persuade him to take it. Fuqua never approached any other actor for the part.
The X and P on soldiers' shields are the Greek letters Chi and Rho, the first letters of the word Christos. The Roman Emperor Constantine had placed it on his soldiers' shields, and it identifies the soldiers as Christians.