Castellan: Castle at War

Strategy Gaming Online Freeware Strategy Games

Castellan: Castle at War is a two-player strategy/war game of castle warfare. It runs on any computer with the Java 2TM runtime installed. As the Castellan you build your own castle, using the landscape to your advantage and minimizing weakpoints. As the Invader you use catapults, siege towers, and battering rams to break into and destroy the castle.
Game play takes place in simultaneous turns, minimizing the time you wait for the other player. You can play against the computer, against another player on one machine, or across the Internet with the aid of a built-in matchmaking system.

http://www.e-brains.com/castellan/download.html