It was a little difficult to follow your text, but I do have these responses.

1. Would this be too simple a combat system.

NO. Simple is best when first making it anyway. Once its built, its far easier to alter/modify a simple system, than a complex. Try it as is. You can always add features later.

- But is it Simple?

- - you are referring to 4 different terrain based values that each unit has strength and weaknesses in.

- - You specified 2 unit types, knights and spearman, but I would assume a whole gamut similar to WarCraft II of foot soldiers, spearman, knights, cavalry, etc... assuming 10+ unit types.

- - You specified terrain types of atleast, but probably more than, 3.

- - The "Simple" you refer to, requires knowledge and understanding of at least 4*10*3, 120 variations that have an effect on a single unit on the battle field, that the player has to manage some how.

- - 120 variations is not what I would call simple.

- That being said, if you are referring to "simple" as the mode of attack, where a unit squad stands in a grid square next to another squad, and wave their weapons to emphasize an attack, no. It is not too simple. There are many popular games that simple say your soldiers went to battle, you lost X blah, and looted X blah.

2. Other ideas.

- I like the basic idea well enough, but I would love to see simple enough things have better effects. I.e. A Catapult actually launches things. Arrows Volley into apposing squads. - - I.e. something that extends this, and makes the effects of fighting spread out farther visually. They can be as cheesy or simple as you want, as long as they are somehow compelling. I.e. Minecraft. Simple cheesy graphics that played out well.

- Regarding the complexity of variations in unit/terrain/attack/move etc, I think it would be a really good idea to look at a strong color palette definition.

- - I.e. Earth bound shows green more, and lower on the units. Water bound shows blue more at the bottom of the units. etc. Better at attack Cities, uses Reds higher on units.

- - Not that those color choices should be chosen in specific, or even to demonstrate that info, but you have so much useful metrics, that if you can start to show it visually, it would be really helpful for the player who just scans the field quickly to get a great idea of what they are up against.