an excellent, if rare, question!
forgive me, as it doesn't come up as often as you'd think,
so I'm condensing what I've read here and at gamers and boardgamegeek,
and what I've seen at local tournaments all into this one thread hehe.
Relentless was ruled via Errata to
apply only for scenario play purposes.
Relentless: This character moves forward only.
Ponderous Relentless/Relentless requires that the ATAT move in the direction opposite to it's starting edge,
much like some GW:W40K style vehicle movement patterns.
this means that, the ATAT can move diagonally, so long as it moves away from the starting edge.
as written, the official "Relentless" means,
the ATAT must not move in a sideways or rearward direction closer to the table edge it started from.
keep in mind, the ATAT is not legal for many squad formats and game modes,
however, it is legal in Dynamic Duos: "Destruction Derby"
and in Dynamic Duos, Tile Wars, and some Epic formats.
The ATAT manual says that it was primarily intended for scenario recreation
also keep in mind,
the Vehicle system and Staggered Activation framework
was never fully completed in WotC StarWarsMinis.
very few customs, and no VSET pieces have developed this framework further,
which means this is an area of opportunity to explore.
the ATAT is also the only character with Relentless and Staggered Activation.
past ad-hoc rulings in Dynamic Duos: Destruction Derby,
have allowed the AT-AT to turn using a 'choose a corner" for the angle and to count squares from,
usually, the corner that you're turning in the direction of - left for left, right for right etc.
with a partial occupied square of the turning ATAT counting as the ATAT for purposes of cover,
movement etc,
and the ATAT only getting AttackOfOpportunity from the front arc
(or sides, if you modify your ATAT to have rappelling stormtroopers ziplining out the sides etc; I recommend the AMT or Fine Molds ATAT models, or a home kitbash rather than hacking up the super-rare WotC mini! [shock])
Stomp means that, any structure with a 'wall tile', must be replaced with damaged terrain tiles if the ATAT walks into that...
this is treated as splash 40.
SWMinis doesn't discern depth or between types of wall and structure very well hehe.
as others have said, SWMinis is not the SWRPG game hehe.
are characters with flight that try to move through a vehicle like an ATAT, such as snow-speeders etc, defeated from the 'collision'?
what about light vehicles that collide with heavier ones? say a rebel troop cart tries to move through a square that counts as the ATAT
because the ATAT is diagonal...
how deep are some pits? some look like quicksand or water... others appear to be space or a bottomless pit,
so as a general rule, Pits are the only impassable terrain for the ATAT.
what if I have melee-reach-2, and I want to attack an ATAT that is turning; can I attack a partial square that has only part of the base of the ATAT and count as adj. for melee?
A: Yup. any square which has a part of an ATAT while that ATAT is not wholly aligned on a grid, counts as the ATAT, so you can attack it. This is probably the 'safest' way to attack an ATAT asymmetrically without the ATAT itself being able to respond.
What if I can only see one partial square of the ATAT behind cover such as a wall or building?
if 3 of 4 lines of site are through a solid wall, with 1 in 4 touching a partial, the ATAT will in that circumstance get cover.
Otherwise, the ATAT's "Colossal" ability, means the ATAT would not ordinarily get cover whatsoever.
*Always get the Arbiter or Adjudicator to verify situations such as these, for fairplay.
nevertheless,
this seems to work very well for most scenarios and situations,
and i look forward to seeing more reflections on this subject.
Vehicles will definitely see more development in SWMinis,
given how popular Tile Wars and Dynamic Duos: Destruction Derby modes are.
