Summary:
Agora is generally considered a marine biased map. From the long hallways to the sheer size of the map, it is easy for marines to capture and hold nodes with relative ease, and it has some easily defendable siege points. But, with its size and abundance of nodes, commandrs need to put continued pressure on the alien nodes, or higher lifeforms will come quickly and in numbers.
Key Points On the Map:
Hans-System 21: This is a popular relocation point for marines. From this location you have access to three resource towers anf can easily hold the onein Marine Start giving four resource towers.
Tunnel RP2: From Tunnel RP2, you can quickly move into Tunnel Hive below and to Cargo hive above.
Main Generator: From this location you can quickly move to Sewer and Tunnel hives.
Siege Positions:
All siege positions below have been combat tested for accuracy and ease of defense. The sieges in the picture below are placed AS FAR AWAY FROM THE HIVE AS POSSIBLE while still reaching.
Cargo Hive:
Siege Point I:
If you are approaching the Cargo hive from the Marine Start side, the hallway above Steam Tunnel is very easy to fortify and protect. You can place the sieges up top at the ladder so that they can be easily protected, and all aliens are bottlenecked at the ladder. There is one vent beside the ladder, but it too can be watched from above. The sieges should be placed as close the the lower wall as possible to give enough room for marines to walk by unhindered.

Siege Point II:
This position is optimal if you are advancing from the south side of the hive location, from Tunnel hive. Most commanders place the turret factory and phase gate against the wall nearest to the hive, but you can place it at the position below to have a farther distance from the door. Try to keep the door closed by left clicking on it. There is a vent directly above this location, as well as one on the wall between The Pit and the hive, so watch for skulks and bile bomb from above.

Tunnel Hive:
Siege Point I:
This location is optimal if moving from Cargo Hive, or The Wedge. Building the phase gate at the tunnel RP2 node and electrifying is really the best way to fortify it, but in a pinch, this location is nice. The sieges can reach the hive from this location and marines can easily move around the sieges to kill anything attacking it. There is a walkway above for skulks to move from the tunnel hive, and two hallways on either side. There is also two vents behind the siege point for gorges, lerks, and skulks to attack from.

Siege Point II:
This location is optimal if moving from the Sewer hive side of the map. Place the phase gate and turret factory on the right wall, that way aliens have to round the corner to attack. the sieges MUST be in the hallway to reach. There is only one way from Tunnel hive, and no vents into the room.

Sewer Hive:
Siege Point I:
Best suited if moving from Marine Start, this is a bit further back than most normal siege locations. You don't even have to weld the water room! Place the turret factory on the left side in the corner, and place the sieges against the lower wall on the ramp. You will notice there is a gap between the siege cannon on the top level and the one on the ramp. That is because that spot causes the turrets to sink, so be sure to leave a bit of room (or be ready to recycle). There is only one way from Sewer, and no vents into the hallway.

Siege Point II:
Most people place the turret factory in the corner closest to the hive, where it is easily bile bombed and swarmed by skulks. By placing the TF further back, you can easily defend and get a ranged attack.

I hope this guide will help you better understand this map, and you can become a better commander and marine. Please, if you know of any added strategies regarding shotgus rushes, relocation, even well used chamber placements and skulk hiding spots, add them. Let's make this a comprehensive strategy for anyone new to commanding to this map.