Ebbtide's defence layer is the one place where the game's central twist - the infected climb using each other as footholds - directly rewrites the usual RTS logic. A wall is a delay and a platform for the enemy, not a wall.
The documented defensive vocabulary is high walls, frontline bases and CIWS. Read them as three layers rather than three alternatives. The high wall defines your position and forces the horde to bunch up at a known point. The frontline base sits forward of it as a forward emplacement, so you are not fighting from a static line. The CIWS provides close-in coverage over the whole defended area. One wall alone is a wall the infected climb. One wall plus CIWS without a forward base is a line you have already surrendered.
In most RTS games, a fortified position is safe because the enemy must path around or through a chokepoint at a known rate. In Ebbtide the mass stacks against the wall and begins forming a ramp out of its own dead and dying. Concentrated automatic fire at the base of that ramp is what collapses it - fire at the top of the wall and you are killing climbers while the pile keeps growing underneath.
Because the horde stacks and climbs, the geometry of your position matters more than the raw number of defenders. A smaller position with a clean kill zone and overlapping fields of fire is worth more than a wide perimeter that spreads your units thin. Make the horde walk into a killing field rather than simply arriving at your wall.
Frontline bases are the most exposed pieces you build and the ones that stop the horde reaching your wall at all. If the horde is chewing through a frontline base, that is a signal to reinforce it rather than to pull the position back - pulling back just gives the mass a clear run at the wall, and the mass is now bigger.
Infected cling to low-hovering helicopters. Helicopters operating out of a base should either stay high or fly with escort suppression, and any helicopter that must descend - to deliver troops, or to pick up survivors - is vulnerable at the moment it does.
Since the tactical map keeps updating while you are zoomed out, you can watch which approach the horde is favouring and adjust the layout before it commits. That is the intended use of the map view - a live planning tool rather than a pause screen - and it is the cheapest defensive tool you have.
Soldiers killed by the horde re-rise as zombies. A position that has already lost a lot of men is a position with a growing problem inside it, and the rubble of your own dead is additional material for the pile. When a position is degrading, reinforce it or abandon it cleanly - do not let it bleed.