Originally posted by behappyandsm1le
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I had some fun testing out options for your first questions. Did not make anything complete, but this should hopefully be helpful to you. I made some pegs in the corners of tiles and an ability that can be used to hammer down any single peg clicked or hammer down all four surrounding pegs if a tile is clicked. Here it is in action:
So how does it work? Well, first I made a peg actor (BP_GA_CornerTower). It is a child of BP_GridActor to enable grid snapping and automatically setting a GridIndex variable. For components it has a static mesh cube scaled to look like a pillar with a blue material. It is parented to the GridAnchor and offset by 100UU in the X and Y axes. The pillar blocks Visibility and no other channels in its collision settings.
At the start of the game this actor adds itself to the GridObjects array in BP_GridManager. It also implements the BPI_Interact interface, which is used by the grid manager to interact with objects in the GridObjects TMap. It has custom events for hovering and ending hovering which changes its material and an event for getting hammered into the ground, which is also called if an interact interface event is called (Pastebin):

Next I made an ability that can be used to interact with the above pillars. It is based on BP_AbilityBase instead of BP_Ability, as BP_Ability has a lot of stuff in it which is mostly useful for unit based abilities, which is not needed here.
This ability basically checks if there are any pillars under the cursor when hovering or clicking. If yes it highlights them (for hovering) and hammers them (for clicking). Hovering or clicking a pillar also prevents the regular grid hovering and interaction code from running. If the player clicks a tile instead of a pillar it will interact with the four surrounding tiles corresponding to the surrounding pillars and hammer them down through GridObjects interaction (Pastebin):

Lastly I added a way to activate this ability by pressing a key (Just a simple way to test it):

Hope this gets you on the right track. I'll see if I have the time to look into multi-unit selection some other day.
Edit: forgot to include the GIF I mad if it in action:

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