Here are listed buildings, as well as important units that can aid in fending off deltas(not necessarily in order of importance):
Naval platform
Guntower
Alligator
Caiman
Armadillo
Dragonfly
Rifleman
Porcupine
Maximum building armor
As delta rushes are normally carried out on sea maps, the standard sea build must be used the while construction boost is active until shipyards are placed.(boosts will be excluded in description). This is to ensure maximum speed in entering the sea, and adequate time to scout and make countermeasures in advance.
construction yard
1 supply center
1 power plant
2-3 barracks
2 supply centers
upgrade headquarters
1 assault vehicle factory
2 shipyards as early as possible
It is then necessary to take resources at sea, while taking note of enemy movements while scouting the enemy base. The earlier the scout, the easier the defence will be.
The many ways of coping with deltas will be listed below.
This is by far the easiest and simplest way of dealing with delta rushes and is self explanatory. When your shipyards are placed, add a naval platform in a good location to guard your coastline. It is instinctive of delta rushers to go for your coast, hence it will be easy to get rid of deltas at your coastline with the aid of some caimans.
In a situation where no naval platform is placed, but 2 shipyards are present, it is possible to somewhat fend off deltas with caimans, since caimans are cheaper and take a shorter time to produce than deltas. If the delta rush is spotted early it is possible to produce enough caimans to fight them off. However, there is the risk of the deltas still coming on land to wreak terror. in this case, it is wise to mine the coasts with 1 layer of mines. Taking out even 2 deltas with mines can be a huge life saver in battle. It is also advised in this case to put a guntower towards the rear of the base.
This can happen in a case where the rush was not spotted early enough and you went for level 2 shipyard(with the safeguard of the naval platform). It is possible that once the enemy scouts and finds your level 2 shipyard, he will most likely go onto land and strike from the ground. Given the level 3 heqdquarters, it is possible to place at least 2 guntowers at the rear of the base. Alligators can be then used to attack the enemy shipyards, or to intercept the deltas before they go onto land. In this case, an armadillo can help by mining the approaches to the base, since deltas cannot remove landmines. If possible, mining the beaching areas for deltas with armadillos also makes a huge difference
This is the nightmare, where the deltas arrive unnanounced, and without you being prepared. The first step must be to place atleast 1 guntower at the rear of the base under a construction boost. Add as many barracks as are allowed by your headquarters level, and produce rifelmen. It is risky to go for grenadiers, because that takes more time and the barracks can easily be destroyed if there are masses of deltas. It is possible to also add an avia factory for dragonflies to support the rifleman. If possible, some sniper towers may help to chip at the deltas since sniper towers are built fast. It is most likely that the enemy may go for vikings, hence the dragonflies will aid against the Vikings. In a case like this, it is advised to start building inland, because if you have nothing at sea, it will be too difficult to repel vikings with ground units. Expand with a construction yard and place a headquarters away from the shore. Add as many extra admin buildings as possible, and use a reconstruction boost when many admin of yours have been killed. Guntowers help on land so it is [possible to expand, but it is necessary to act fast.
It is always recommended add a naval platform when deploying the base for sea
Scout on time
If deltas approach, add guntowers to the rear of the base
Produce whatever units possible to cushion the attack
Shift the base if possible/necessary
Act quickly and decisively, and without panic