Realm of Battle
The original "Citadel Realm of Battle Gameboard" set contains six such squares, four of which have quarter-hills (which can be arranged together in a large central hill, or in half-hills at the edges of the board, or quarter-hills in the corners) and two of which are flat. All of the tiles are slightly textured with cracked earth in places (which can be decorated as exposed, weathered stone, or scorch marks, or whatever). GW later released an "expansion pack" consisting of two flat tiles. Anyone who buys two flat pieces of plastic is a fool -- in fact, anyone who buys two flat pieces of plastic and four flat pieces of plastic with a plastic quarter-hill in one corner is also a fool.
Forge World has released some terrain tiles of their own, which are much more detailed and harder to scratch-build and thus more worthy of consideration (and also more expensive, but that should not be a surprise).
The first is the "Imperial Strongpoint," which is edge-compatible with the Citadel kit's quarter-hills, but is shaped on the interior like the opening to a bunker, with pillboxes, a generator, and some dragon's-teeth tank traps in front of a set of blast doors leading into a hill.
Later, for their Imperial Armour volumes focusing on the Badab War, FW released four "Zone Mortalis" tiles (designated Alpha, Beta, Gamma, and Delta), which are flat 1-foot-by-1-foot tiles with walls arranged in different patterns. The idea is that they can be assembled into mazes to represent the cramped environment of boarding missions, Space Hulks, bunker interiors, and other such situations. Modular mazes where everything is at right angles are relatively straightforward to scratch-build, but FW did a great job on the texturing and detail (Imperial iconography only, though -- sorry, xenos and Chaos players). They later expanded the line with Epsilon, Zeta, Eta, and Theta patterns, as well as little doors, desks, and other (Imperial-looking) scenery bits to spice things up.
As part of the first wave of Horus Heresy releases, FW produced four "Cityscape" tiles, with detailed, rubble-strewn streets, wrecked vehicles, industrial-looking bits, and slightly-raised platforms for buildings and other scenery. Again, they're expensive, but they look great (like everything else Forge World makes). They later released a crashed-Thunderhawk tile and a Space Marine fortress. As part of the lead-up to the release of The Horus Heresy Book 3: Extermination, in March 2014, they announced the "Generatorum Nexus" (big ol' half-exposed generator turbines and dynamos) and "Manufactorum Sector" tiles (some half-sunken industrial buildings that take up space but don't have flat tops, so models can't stand on them).
For Imperial Armour Volume Twelve, Forge World produced a Necron-themed tile, the Tomb Citadel. It depicts a ziggurat-like structure, with layers and steps rising to a step pyramid in the corner. Said pyramid has an indentation on top that permits a Monolith to rest there.
- Zone Mortalis video on Youtube
- Cityscape video on Youtube Which, for some reason, has a fucking DEATH COMPANY CONTEMPTOR DREADNOUGHT on it
Fans of Age of Sigmar, feel something vaguely comparable to a positive emotion!