At the moment the game allows you to fight and conquer territories in the British Isles and Ireland.
The list of territories can be found in the All Territories section. These are:
Aberdeen - Anglesey - Argyll/Antrim - Buckinghamshire - Caerphilly - Carmarthenshire - Cheshire - Clare - Cork - Cornwall - Cumbria - Denbighshire - Devon - Dorset - Down - East Ayrshire - Essex - Galway - Gloucestershire- Gwynedd - Hampshire - Highland - Isle of Man - Isle of Wight - Kent - Lanark - Lincolnshire - Longford - Mayo - Meath - Merthyr Tydfil - Moray - Norfolk - Northamptonshire - Northumberland - Nottingham - Offaly - Orkney - Perth - Powys - Scottish Borders - Somerset - Staffordshire - Tyrone - West Sussex - Wexford - Wiltshire - Yorkshire - Zetland
Once your army is big enough you can go and Attack Territories.
You can also choose to Reinforce Territories instead - whether these are yours or under the control of other players in your Kingdom.