Architect magic 

When you reach certain levels of this subclass, you gain certain spells based off your level. You always have these spells prepared and they don't count against the amount of spells you know.

Artificer level    Spells

3                          Magic missile, Sanctuary

5                           Lemond's tiny hut, Otiluke's resilient sphere

9                           Summon construct, Wall of stone

15                         Creation, Transmute rock

Tools of the trade

When you chose this subclass at 3rd level, you gain proficiency in carpenter's tools. If you already have this proficiency, you gain proficiency in one other artisan tool of your choice.

Defensive structure 

Starting at 3rd level, during a short or long rest, you can use your carpenter's tools to build a structure for you and your companions. This structure can hold 10 small creatures, 5 medium, or 2 large. While inside, they get the benefits of the sanctuary spell. If you build it during a long rest, after you and your companions finish a long rest in it, you all gain 1d4 temporary hit points. These temporary hit points increase at 5th level (2d4), 9th level (3d4) and 15th level (4d4). This structure loses its magic after 1 day has passed. Once you build one of these structures, you can't do so until 3 days have passed.

Acrcitectual barrier

Starting at 5th level, whenever you use your Carpender's tools as a spellcasting focus, you can use your bonus action to give yourself resistance to bludgeoning, piercing, or slashing damage (your choice) from nonmagical attacks.  

Offensive castle

Starting at 9th level, you can build an offensive castle over a long rest with your carpenter's tools . It can hold 40 small creatures, 20 medium, or 10 large. It has 5 turrets that when an enemy reaches in their range, the range of magic missile, they cast magic missile as a 3rd level spell. They can do this 5 times, and then regain the ability to do so next dawn. The castle last 3 days. After you build a castle, you can't so again until 5 days have passed.

Improved building  

Starting at 15th level, when you make an offensive castle, it also gain the benefits of a long rest defensive structure.  Also, when you use your Acrcitectual barrier, you also gain the benefits of half cover for the same duritation.