With the addition of allies and the ability to purchase a fortification, certain tactical options now present themselves that weren't always available in 5th edition.
For example, if you have an important unit in the game that you need to keep alive, "Castling" is now a viable option. There are a number of units such as these: Eldar Farseer with runes of warding for psychic defence, and Epidemius for a buff to all Nurgle units. These guys don't really want to see the front line of combat if they can help it -- particularly if they're simply an allies choice in a force composed of other primary elements who ARE there to get to the front lines.
So, here's the idea. As well as your valuable allies that provide you with nifty bonuses, you're going to need somewhere for them to skulk away and hide in. What better than a bastion? With AV=14 at a third of the cost of a land raider, why not? The bastion will take a bit of pounding to get at, and it is probably not going to be target number one on your enemies' hit lists. Or at least, that's the theory.
Let's have a look at Epidemius. To Castle Epidemius is relatively simple. Because he is going to be deep-striking through daemonic assault on to the battlefield, all that we need to do is have an icon inside the bastion at set-up. When Epidemius arrives (hopefully on turn 1!), he will deep-strike without scatter to the ground level of the bastion, and within 2 inches of the front door (for the rules for embarkation to work). In the movement phase, disembark the marines (or whoever it was with the icon), or perhaps move them up to the battlements section, and then in the shooting phase, run Epidemius in to the building. Castling move complete!
You now have Epidemius and chums safely tucked away and are ready to take on the rest of the battlefield!