1.Murderholes, 'nough said. (Functional gatehouse)

1. Big strong gates with a big beam across. Just the feel of a town/castle etc. When they push and shut the gate.

2. Machicolations, a bit of an extending of murder holes.

3. Bastions/Towers, with chest-height to head-height parapets, so it'll be harder for enemy archers to hit you.

4. Moats, preferably dig-able, and fillable with water :P

What I listed are not just buildings, they're buildings that allow a special kinda tactic to play out.

That said, poisoning a well or throwing in dead corpses (using poison and plague as tactics) could be quite interesting.

Typical military formations would be fun, especially for foot soldiers. This could turn into an advanced rock-paper-scissors similar to many strategy games. For example, packing your infantry into dense pike formations would give enemy cavalry a tough time, while shield walls would protect you from archers. You could create phalanx tactics to counter both at once, but these are vulnerable on the sides to more mobile formations.

These and other tactics could of course be varied depending on terrain, weather and a variety of factors to create a RTS element that's easy to learn but very difficult to master.

Animal poisons. - poison pigs or cattle in an area so when they are slaughtered and consumed they poison the person eating the meat.

Diseased catapults - fling dead diseased corpses over castle walls to infect those within.

Fouling water sources - being able to remove a water source from a settlement so they no longer get the town buff and people start to die from thirst.

I'm short I hope for some form of indirect combat. Most games deliver in your face pvp but I would love to see a way for "special forces" to sneak into a kingdom and sabotage or even wipe out a larger force if they aren't prepared.