You may get confused by the title of this paragraph. But really, there is no reason to do so. These terms are used interchangeable in communication. These tems are however only refering to the homekeeps of each alliance. The Ebonheart Pact has 'home keeps', the Daggerfall Covenant has 'home forts' and the Aldmeri Dominion has 'home castles'. For the sake of the article we will refer to all of the mentioned as 'keeps'.
The first objective of each alliance in the three banners war is to capture and hold their home keeps. Owning them will grant all the players of that alliance an xp, gold and ap earned bonus. This bonus applies to all activities in Cyrodiil. Whether you clear a dungeon, capture a keep, kill an enemy player or hand in a quest, this home keep bonus applies. Each alliance therefore has their own list of keeps functioning as home keeps.
In order to capture a keep (be it a home keep or an enemy keep), the first topic that comes to mind is sieging. Laying siege to keeps is a core mechanic of pvp in eso. The objective of sieging is to breach the inner and outer walls or main gate. Attacking a wall is very different from attacking a main gate. Each approach has its own risks and benefits. Both the attacking alliance and defending alliance have a limitation to place up to 20 sieges each. Once a wall or gate drops below 50% health, the keep will be contested. Which disconnects the keep from the transitus teleporting system and disallows respawning at the keep.
Situation 1: the main gate
The main gate is the fastest way of breaching the keep. However it comes at a big risk. The benefit of attacking the main gate is the low hp of the gate compared to walls. Also the main gate can be breached quicker by placing a siege ram. The ram must have at least 3 occupants for it to function and operates at full damage potential when 6 people occupy the ram. A common tactic is to first place a stone ballista up close in front of the gate and let the ram ride into the same position. Optimizing the space available and leaving room for placement of multiple stone ballistae in front of the gate.
Eventhough it is the fastest way of entering a keep, attacking the main gate however comes at a big risk. The main gate allows the defenders to place multiple oil pots above the entry way. Dealing massive damage over time to both the players and siege weapons below. Also upgraded keeps offer positions for the defenders to shoot from the sides.
Situation 2: the walls
Sieging walls may take a bit longer but are (generally speaking) harder to defend. Often the still standing walls offer protection from above next to beached walls. Players used to siege these walls with stone trebuchets, but since the Homestead patch it is more effective to siege using stone ballistae. Since they can be placed closer together and do approx. 6k damage per rotation with three ballisae used by one player. Compared to the time it takes to shoot one stone trebuchet, which deals 4k damage per shot and take up more space.
Attacking a wall is slower than the main gate, but has one significant benefit. The distance between the outer keep walls and inner keep walls allows players to siege both of them at the same time. This tactic requires players to place stone trebuchets close to the outer keep wall, focussing the inner keep wall. While the remaining players siege the outer keep walls. Timing this well, a.k.a. taking both walls down at the same rate, will make an easy entry possible and reduces the risks and time it takes to enter an enemy keep.
Each keep consists out of two main areas: The inner keep and outer keep. In order to enter a keep, both the inner and outer walls or gates have to be breached. Eventhough there may be no players inside the enemy keep defending, NPC's are awaiting your arrival with great patience. The outer keep has NPC's defending the main gates and corners of the walls. The inner keep is defended by flag NPC's and NPC's on the transitus shrine. These NPC's are no challenge in most situations, unless you play on the Azura (no-cp) campaign. Inside the inner keep you will find two flags: the nave and apse flags.
Once you have given yourself access to the keep and killed most of the enemy defenders and NPC's, it is time to flip the flags. The nave flag (often refered to as main gate flag) is located behind the main gate. The apse flag (often refered to as the back flag) is located behind the nave flag in the keep. A common tactic is to rush over the nave flag and take the apse flag first. Mostly because the nave flag seems to have the most positions to set up defensive siege (such as oil pots) and because most groups prefer to clear the NPC's quickly, since they also place negates and damage aoe's.
Once your alliance has more players on the flag than the defending alliance (NPC's also count towards this) the flag will flip colours. You have to turn both the nave and apse flag in order to flip the keep to your alliance. Once the keep is flipped it will be uncontested unless the enemy hits the breaches you made in your offense, while under the 50% treshold.
After capturing a keep it is of great importance to repair the keep. Since you probably breached both the outer and inner walls or gates, you will have to repair both. Note that if a destroyed or damaged wall is hit by an enemy siege weapon, you cannot repair. To repair walls or gates you need to buy consumables at the merchant inside the keep, outpost or at the alliance gates (e.g. Northern Morrowind Gate for the Ebonheart Pact). For walls you need keep wall masonry repair kits and for gates you need keep door woodwork repair kits. In order to use them you need to put them in your quickslot.