"[T]his wicked Tower, like a cruel giant in a fairy-tale, must be fed with blood, and that blood must be the best and bravest in England, or it's not good enough for the old Blunderbore."
— William Schwenk Gilbert, British librettist, The Yeomen of the Guard
Based off the Yeomen Warders created by Henry VII Tudor to guard the Tower of London in 1485, Tower Warders are available only from the White Tower wonder once you have researched Centralisation, either from your Nobles' Court or from the Tower itself, and resemble the Frankish-styled Halberdiers, but with two exceptions: the first is that they cost wealth and metal as opposed to food and wood, and while they are created much faster than normal light infantry are, they have exorbitant ramp costs. Nevertheless, these men perform as well as the Halberdiers of their Continental foes do on the field, and can be summoned fairly quickly - much more quickly than the average espadachine or halberdier, making them a great defensive unit for a commerce-minded faction.
So while Tower Warders can be recruited by any of the four factions capable of erecting the White Tower wonder, it is the English and the Scots that will often find these units worthwhile, as the English can use them in place of their cheaper yet slower-training Knights-Sergeants if the need should arise; the Scots will find them a most welcome addition to an infantry-poor late game army whenever so needed (being capable only of creating militia, cavalry and javelin men). Else, these units will have to remain as a purely defensive unit which shouldn't be created unless the enemy is already near the very gates of your town.