Invasion, a Campaign, is a Story. The Story comes alive against the backdrop created by the Aesthetics - the aggregate of the Artistic Elements of the forge work. Aesthetics are used to draw the Players into the Game Play by creating an engaging atmosphere. A uniform theme, ambiance, and architecture running through out the map is critical to making a believable and immersing Story. And the architectural structures in combination with the Objectives make the progression of the Objectives flow logically to form the premise of the Campaign.
All of the Artistic Elements forged onto the map make up the backdrop for the Story. The structures, facilities, and grounds being invaded; the colors, architecture, paths, and elevations - they all play a part and must fit together to form a uniform theme of the location being invaded. And it is important that the Aesthetics do not bore the Players. All the great forging is for nothing if the Players are not excited by the feel of the map as a whole.
If the theme or location has elements that clash with each other, then the Players can lose the immersing aspect of the Game Play - they can find the Game as just another map made of blocks, rather than a place with any solid theme. Cohesion between the various structures and path ways is necessary to make the entire map look like one map.
The Defenders' strong hold must reflect their Species. However, the Forge World palette is centered around the Forerunner architecture. It seems reasonable that Elites are more likely to defend a true Forerunner facility, and Spartans are exclusively defenders of Spartan facilities.
The Objectives must stick out and grab the Players' attention, but not in a negative way. Instead of grabbing a Player's attention by sticking out and breaking immersion, Objectives need to look like they are part of the surrounding backdrop, but stick out to grab attention as those parts of the map that a Player naturally should draw toward.
Using contrast in color or detail can help draw a Player's attention toward Objectives. Lighting up the Objective in a dark area of the map, or increasing the fine detail of the Objectives with switches and knobs are both examples of contrast. When a Player walks around the map, the Objectives should stand out as something they would naturally be drawn toward - at the very least. Ideally, a Player should be drawn toward the Objective simply because it looks like it is important - like it has a purpose in the Story.
Aesthetics across the backdrop must come together to help make that Story believable and immersing.
The strong hold of the Defender must yield strong cohesion with the Defenders' Species.
Objectives need to be naturally obvious - even without the Objective Zone present; and using contrast can help draw players's attention, standing out as obvious Objectives.