I'm somewhat a fan of static defence and fortress warfare. Coincidentally this is almost always the way the Imperium fights Tyranids on the ground when they have the luxury of choosing their battlefield. So, how do you guys like the idea of an Imperial Bastion that the Ultramarines must defend and the Tyranids seek to breach? You could have several points to most advantageously assault the rampart from, with different traits. Such as a quick route, with the disadvantage of it being easily defended. A long route with plenty of cover on the way, but the longer route means the more time for the Ultramarines to shoot, a secret path that can be discovered and leads into some kind of basement, etc.
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Woah, I got so hyped that I went away and made a map and then came back to find this unfinished post! Convenient eh?

So, here is my very rudimentary Paint diagram and I shall explain it! Hopefully it catches the attention of one of you glorious mappers!
BiggerOkay, first the basics!
Grey: Walls and inaccessible/impassible areas.
Light Grey: Ground floor accessible areas.
Dark Grey: Raised areas, presumably battlement defensive position. Also stairs.
Dark green: Gates!
Light Green: Accessible areas.
Black: Bottomless pit of doom.
Brown: Ridges, mountains, any impassable terrain obstacles .
Blue Cross: Ultramarine spawn point.
Blue Dots: Just points showing where they would defend from on the battlements above.
Red Cross: Yeah, you got it, Tyranid spawn point.
Now on to the points!
A: Ultramarine spawn point. This could possible be a Thunderhawk landing pad or gateway leading from somewhere. Anything plausible. I personally like the landing pad idea. Those grey walls you see around it don't necessarily have to be high walls, they can simply be stonecrete defensive barricades. The point is, if the Space Marines are pushed back, that is their last stand point. I'm not sure how the game mechanics work, but presumably if the Tyranids are within the spawn area for an extended duration of time they win.
B: Front gate set into the wall. I know that in hammer you can make breakable objects. If the door were to be custom made it could be done so to withstand a certain amount of punishment before breaking. Smaller defensive positions could be placed behind the gate, in the archway under the wall. On top of the gate would be the fortifications where the brave Ultramarines would rain explosive bolts down upon the xenos foes besieging them.
C: Can you decipher those bizarre shapes? Stairs, aye, stairs.
D: Defensive position number two! The battlement space would be considerably smaller, but offer a more advantageous firing position on any who seek to batter down the gate. The gate could be made to endure less punishment than the main one for balancing issues.
E: The chamber that the second gate leads into. This space could serve as a rallying point for Tyranids before they break through another gate which leads into the main fortress area. Small fortifications, like barricades, could be placed around that gate to offer the Ultramarines another chance to repel the foul Xenos as they seek to infest the bastion proper.
F: This is where the close quarter, desperate combat kicks in. That area of grey squiggles represents a destroyed area of the bastion, a maze of crumbling walls and easy ambush spots around each corner. If the Tyranids fight their way through the ruins they will find themselves bereft of incoming fire from high up, as this gate has no battlement platform for the Ultramarines to fire from.
G: A simple terrain obstacle to distinguish between different assault routes, this could make a nice gorge of unfathomable proportions!
H: At last, the spawning point of the ravenous Tyranids. As this won't be an area that will see any fighting, as the Ultramarines have no purpose going there, it can be a simple work of aesthetic taste. You know, that wonderful fauna the Tyranids tend to mutate. It should lead clearly to the three paths however, unless you wish to keep the route that leads to
E a secret to be discovered, in which case you would have to make the entrance to the route hidden.
Things to consider:
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Balance: If done wrong, this could swing horribly either way. The fort can't be too easy to hold, or too easy to breach. It has to be done so that victory hinges on player skill.
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Conditions of Victory: How does either side triumph? Perhaps the Tyranids have to breach the spawning point of the Space Marines while they have to hold the fort for so long?
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Cover: The placing of cover and fortifications is somewhat vital on this map. Depending where you place it, it can swing the usability of the paths. Will you put plenty of cover on the way to the main gate, or leave it with none and place it along the secret path?
So, sorry to bury you with those details! What do you think? Any takers?