From Halopedia, the Halo wiki
Valhalla is a multiplayer map in Halo 3 and Halo Online, known as Riverworld in the latter. The map is set on the Ark. Structurally, it is reminiscent of Blood Gulch from Halo: Combat Evolved. The map is a sparsely forested valley with mountains in the distance, featuring a river, rocky outcroppings, hills and a large Forerunner-built wall.
Valhalla is set on the Ark. Structures similar to this canyon are echoed on many Forerunner installations including the Halo Array and Shield worlds, with the site known as "Two Giants" of particular note on Requiem. Buried within deep canyons, these towers are not only sheltered from off-world debris, but their positions allowed them to leverage the steep environment's natural harmonics to amplify their signals when firing deep into space. Most scientists believe the spires - at their base function - act as communication channels, sending specific commands to other remote locations. The full operation and overarching purpose of these mysterious objects, however, remains a source of speculation. Canyons like these were often reused based on the specific tastes of their creator, their function and as the result of other semi-random variables in the chosen design template.
The map is similar in shape to Blood Gulch but has unique elements of its own. Valhalla is not a remake of Coagulation, nor is the downloadable Standoff either. Valhalla also has features from the Halo: Combat Evolved level, Halo. The most obvious feature is both the river and the bases themselves. The river starts in the mountains behind the blue base, and comes out as a waterfall. The river then flows in front of the base, under an outcrop, then down between the two prominent hills on which players find the Spartan Laser and the machine gun turret. Finally, it bends around the base, and empties into a large lake. Also, there are two caves in the map: one that goes through the hill between the crashed Pelican and blue base, and another that goes through the hill between the Pelican and the red base. Do note that if weapons are dropped into the river, they might drift downstream.
The distinguished beam emitter bases are Forerunner technology. They are similar to the structures seen in the Halo: Combat Evolved for PC multiplayer map Infinity, Timberland and in the campaign level Halo, while the lower portion is reminiscent of the bases in Blood Gulch. There are three entrances to the inside of the base; two on the sides as seen in many screenshots, and another located behind that dips down into the base. Replacing the teleporters are the man cannons. The largest one, on the front of the base, will send you out a third of the way out into the main battle area, while the smaller ones on the side of each base send you a short distance to the nearby cliff for a quick getaway. The bases feature a shiny glass floor on the interior, allowing players on opposite sides of the floor to see each other.
As previously stated, there are two similar hills on either side of the river. On the higher one spawns the Spartan Laser, while the lower hill holds a machine gun turret. Each base holds a sniper rifle inside the rear entrance, a pair of Fragmentation Grenades, and a pair of plasma grenades. Numerous battle rifles spawn around the map: two on top of the base next to the smaller man cannons, two underneath the base alongside each entrance along with two plasma pistols, two on the terrain where you land after taking off from the man cannons, and one that spawns in each cave. A shotgun spawns inside each of the caves as well, and Spikers spawn on the top of both bases near the Missile Pod. A pair SMGs also spawn on each base and are located next to the plasma grenades. Valhalla is a very good sniper map since you can snipe opponents clear across the other side of the map, and when players take the bases' man cannons for rapid transportation, they become easy targets for experienced snipers.
The Missile Pod spawns on top of each base to provide initial anti-vehicle defense. The vehicle set per base is a Banshee, Warthog, Wraith, and two Mongooses. However, in the Matchmaking playlists the vehicles are limited to a Warthog, a pair of Mongooses, and a Banshee per base (in 4v4 matches, most notably in Team BRs, the Banshee is sometimes removed). The power drain spawns on top of each base next to the Missile Pod, while the bubble shield spawns at the neutral Mongoose near the Forerunner wall.
There are also points on the top of the spires that can be reached by Banshee, Hornet, or in Forge, but these areas will not be listed.