Codex Martialis, is basically a stand alone core rules system; or can be used as a suppliment to any existing 3.5 OGL system (Pathfinder, D&D, etc).
While the core rules are light, consisting mostly of the combat rules. It is adaptable to most genre's or settings a GM could want. The Combat Rules are an adaptation on the d20 system; with an emphisis on making combat more historically accurate.
Taking references from historical texts of famous fighters, the combat system doesn't add a level of complexity; that will slow down the game. Starting with a Martial Pool, players are able to draw from the Martial Pool (MP) during combat. The MP allows GM/players to make the usual attacks and defending. However it also allows players/GM's, to use the MP; to activate special (weapon) feats as well as counterattack.
How it achieves this, is to use a number of d20's; dependant on the action required (max of 4d20). To attack, it migh be 2d20; with only the highest d20 counting. So a player with only 2d20's, could use those dice for one attack; then defend, use a feat or counterattack.
It lacks a focus, for it to be a true core rules. However, it does work really well; as a suppliment to exisiting 3.5 OGL systems. Adding Codex Martialis to a campaign would be simple enough. All the existing feats and weapons in 3.5, have been included in this. Any feats that have been replaced, have been noted; as well as comprehensive details of how new feats work.
Overall a great publication, that will allow a GM to add an extra flavour to their campaign; using real fighting techniques from history.
[5 of 5 Stars!]