There are 3 main sources of reinforcements for the battleground.
1 is the capital city
2 are the forts
Troops spawn in these places and walk to the battlefield in squads to help fight.
When a fort is sieged firstly the Orcs move a large army and some siege weapons to surround the fort. The Orc army is made large enough that the spawned troops from the fort could not break through it (the fort is "sieged" which means nothing coming in or out). It may be a good idea to tag along a "boss" of some kind with the sieging army that is used as a factor to break the siege. In response to a siege the fort stops producing spawns for the battle ground and closes gates and uses defensive siege weapons of its own. The Orc army will periodically receive reinforcements making it stronger and replacing any lost units. The fort is given very limited ability to replace lost units such as a fixed number of replacement units before it is no longer able to replace units.
Sieges go through a few phases.
1. When a siege starts, the fort deploys units behind gates ready for combat. The Orc siege weapons then start firing at the fort siege weapons (which are probably towers?). The fort siege weapons will likewise fire back at the enemies (not necessarily the siege weapons). The fortress siege weapons are made so that they deal reduced damage to the Orc siege weapons but decent damage to the troops so that the fortress siege weapons alone are not able to break the siege but can help when breaking the siege by killing units. The Orc forces stand by the siege weapons waiting for reinforcements. This is designed to be the easiest phase for a player to break a siege, before the Orcs get settled in.
2. Over time reinforcements for the Orcs arrive. The fortress siege weapons are also likely low on health if not dead so the siege weapons start firing on the fortress troops (which take reduced damage so they do not die too fast). After the first few waves of reinforcements (3 sounds like a good number) they launch an attack against the fort. This involves them attacking the gate while either side have their range units exchange fire. The fortress might also have a number of helping towers to turn the defensive odds in favour of the fortress this time round. The fort should hold back the first attack with the Orcs pulling back after some losses. The fortress replaces lost troops (to a certain degree).
3. Orc reinforcements start to kick up tone with larger squads coming in. The fortress should also be suffering from the siege weapon bombardment. The Orcs launch another attack against the fortress (which probably has no gates now) which, if un-hindered, will take the fortress. This time the attack is headed by the Boss unit (to be extra sure of success) although he should retreat if put below 33% health. If the Orcs take too many losses (maybe due to player fighting outside before the attack) they pull back for a final time.
4. A last large wave of reinforcements arrive for the Orcs. All units in the Fortress are given "No Hope" (a representation of starvation, illness and mental state caused by prolonged siege) that lowers their damage, causes them to take damage over time and take more damage. The Orcs now send everything they have at the fort in a suicide attack (no retreat). The Orc siege weapons are sent away (recycle in a later siege or whatever) as this represents a final charge to take the fort. The fortress itself will offer little resistance so will easily fall to the Orcs. If a player somehow turns the battle around (maybe the number of troops is very small due to constant harassment of the Orcs) then the siege is lifted and the fort holds.
When a siege is lifted, the fortress sets about replenishing any garrisons, rebuilding siege weapons/towers and then sending more troops to the battle field.
The fortress falls if a central building (castle?) is destroyed. The castle itself cannot be killed be the Orc siege weapons (maybe only weakened?) and requires actual forces to be nearby to be destroyed.
The siege is lifted in 2 ways.
1. All siege weapons and the Boss unit is dead.
2. Phase 4 complets and the fortress has not fallen.
When a siege is lifted, all remaining idle Orc troops and reinforcing Orc troops run away. Orc troops that are attacking the fortress keep attacking (unless they go into retreat mode). This presents a nice time to attack using the player heroes during Phase 2 as the Orc Boss unit and siege weapons have very few troops around them.
To help the players lift a siege there are a number of tactical elements available. Firstly are their heroes, strong fighting units capable of killing many Orc units and taking down the boss unit. They can also order a "counter attack" from the fortress itself which causes all units inside the fortress to charge out and help lift the siege. Finally if the siege enters phase 2 and above they can call for help from the other fortress (if alive) which will divert troops from the battle field to form a attack squad (which the player should support to help lift the siege).
The siege should be impossible to lift without player help. Ordering a counter attack or calling for help leaves the troops "Leaderless" unless a player is actively near the siege area. Leaderless is a powerful debuff that greatly lowers unit damage and durability so that the Orc siege force can effortlessly hold them off.
The idea is for players to work on lifting sieges as soon as possible. A siege can be lifted very easily by flanking the Orc siege forces with a hero and ordering a counter attack if done early enough. Waiting too long makes it become harder, however a well timed call for help and counter attack with hero support makes short work of it. If the player takes too long to lift a siege, the Orc siege forces start to become very large, the fortress very weak and more difficult to stop falling. Eventually the fortress offers little resistance and falls as a penalty for the player taking too long to help (it should fall eventually without any aid).
A key factor is player evolvement in lifting a siege. The penalty on counter attack and call for help troops is to stop a player solving a siege by a few simple command. His hero will actually need to be in the fight for the troops to have any actual effect.
Another possible way for the player to handle a siege is to use the Wave 2 and 3 charges and defend the fortress from the inside with powerful abilities. Having a player inside could boost the units defence and damage with a "High Moral" buff. After successfully holding off an attack a counter attack could be launched (again with buffed troops) to life the siege. To get a player into the fortress during a siege, a mage in the capital could "teleport" him into the fortress.
Difficulty can increase Orc troop sizes, boss unit strength and number of siege weapons.
