Dark and Light has Full Loot, Free-for-all PvP server(s). Official servers are designated "knight" in the name to identify their PvP Servers. Unofficial servers will, usually, have PvP, RP-PVP, or PVPVE in their server title.
There are no subset rules at launch of early access (e.g. no official PvP-RP server).
The only other confirmed server ruleset is PvE1) .
PvP is not possible with your own faction in your faction capital cities2) 3) .
It is possible to engage players of opposing factions in your capital cities. There are guards who are "pretty strong" 4) at least in early game, but frequently get wiped by high-level tames and players.
In the wilderness, it is a free-for-all.
There is a formal siege mechanic through the combination of House Seal (defenders) and Siege Flag (attackers). It was also possible to raid bases unprotected by House Seals. However, building strength has been substantially buffed in a patch dating7). This impacts on the ability to successfully siege, particularly high-tier bases.