Brink

For this achievement you have to complete the Founders Tower map from both sides of the conflict. The Missions can be accessed like normal from the campaign menu.

The following guide is based you playing with a few people you can co-ordinate with, as that makes it much easier. If you don't it's still ok. Just don't rely on the bots as they are designed to NOT complete objectives so if you are lacking human team mates always switch to the class that is needed.
It's also a good idea to presume that everyone on your team is an idiot so you can anticipate being ditched by your team and be prepared to do things alone for a bit. (E.g Use guns with plenty of ammo like the machine guns, don't use the snipers because these are useless when you find yourself out numbered.)

RESISTANCE : Day 23 Tower Defense

The first objective is easiest to defend from the balcony just outside.
If you feel it is adequately defended you can go down the stairs from the adjacent balcony (Past the sealed door, you'll have to go down and back up to get to it.) and join what should become fighting outside. if you do this though beware of enemies making it through the fighting and round the side. As that passageway leads directly to the objective and once attackers are in there they are hard to kick out.

The second and third spot harder to defend but still quite achievable if you have a good team that stick to some good defensive positions near the objectives.

The forth can feel more like an attacking objective than a defensive one because a good security team can easily make it to the objective before you do so it's a good idea to get to the objective with a sizeable group of people so you can lock down the point earlier  to tip the balance back in your favour.

The last objective is to stop  the bag runner reaching their objctive a short distance away. It's important to lock down the bag before they reach the scaffolding because that is near where the security spawn and can be very easy to get overwhelmed.

SECURITY : Day 23 Founders Keepers

Standard tips for attacking apply.
- Make sure you have plenty of each class on your team and switch as often as needed.  
- Stay in large groups to make it easier to overwhelm the enemy.
- Try to attack from numerous directions at once to confuse the enemy.

For the first objective it's a good idea to go along the side passage way that is to the right of the bronze beams that run outside shortly after you spawn. The first objective is the hardest but is easy to defend so if you can get a group of about 3 people in the room it should become quite easy.

The other objectives are the standard fare. When going for the second objective stay off the bottom floor as you can be hit by the sentry guns guarding the spawn points. And after the third objective make a dash to the next one. It's a lot easier if you can get there before the enemy do.

~#~ Towering Achievement ~#~

This achievement requires you to win the new DLC map "Founders Tower" as both SECURITY and RESISTANCE factions, in CAMPAIGN mode. Select "Campaign" from the main menu and select the mission "Tower Defence" for RESISTANCE and "Founders Keepers" for SECURITY. Upon completion of both of these missions, your achievement should pop.

Campaign mode is significantly easier when done with at least one friend. If you have trouble, I have included some general tips for success in Campaign mode.

~ Your bots are much less effective than their bots. This is an unfortunate fact. Learning to deal with your bots ineptitude is am acquired skill. There are a few things you need to keep in mind.

~ The bots are actually programmed to be disenclined to perform Primary Objectives. If you aren't the Soldier planting the bomb, you'll need to stand near the planting spot for a good 30 seconds or so before a Soldier bot even thinks about planting it. It's intentionally programmed that way to stop the bots playing the game for you.

Best solution? Just always be the primary class.