The Complete Total War Unit Guide.

3. The Viking Invasion

The Viking invasion is the add-on for Medieval: Total War. It contains a new campaign set during the Viking invasions of Britain, because of this it has a whole new set of units for this era as well as some new units for the regular medieval campaign. None of the new units fall under a new class so I will not detail the advantages and disadvantages of each category, to see that just scroll up to the same category in the MTW section.  

A note on unit availability: I have noted when a unit is available for the player to build in the two campaigns. The unit is also available to custom and multiplayer battles set in that era. Saxon huscarles (and other similar units) are noted as being Viking only, this is because you cannot build them in the medieval campaign even though they sometimes appear in rebellions in the medieval campaign.

3.1 New Spearmen
    *Armoured spearmen
    *Dismounted nobles
    *Fyrdmen
    *Royal bodyguard (infantry version)
    *Round shield spearmen
    *Rus spearmen
3.2 New Swordsmen
    *Arab infantry
    *Berserkers
    *Celtic warriors
    *Dismounted faris
    *Jobbagy
    *Swabian swordsmen
    *Slav warriors
    *Viking Carls
3.3 New Heavy Cavalry
    *Avar nobles
    *Byzantine lancers
    *Khazar royal cavalry
    *Mounted nobles
    *Royal bodyguards (mounted version)
3.4 New Light Cavalry
    *Druzhina cavalry
    *Horsemen
    *Pictish cavalry
    *Viking raider cavalry
3.5 New Missile Cavalry
    *Faris
    *Steppe heavy cavalry    
    *Szekely
    *Pictish mounted crossbows
3.6 New Axemen
    *Saxon huscarles
    *Viking landsmenn
    *Viking huscarles
    *Joms Vikings
3.7 New Peasants
    *Viking thralls
3.8 New javelin units
    *Jobbagy
    *Slav javelin men
    *Spanish javelin men
    *Bonnachts
3.9 New missile units
    *Irish dartmen
    *Pictish crossbows
    *Sherwood foresters
    *Welsh bandits
3.10 New Artillery
    *Organ gun