Most of the photos you can find from the great wall has arrow slits on both sides. In certain portion of the great wall, smaller arrow slits are facing inside only. The outfacing side of the wall usually have bigger wider diamond shaped wall slits that allow soldiers to scout with bigger field of views and shoot enemies, and in certain location of the great wall out facing watch slit even equipped with stationary steel canons. While the smaller arrow slits usually don't have watch slits on top of them, the wall above the arrow slits are usually flat. Because there's no need to watch for invaders coming from the inside. The Great Walls were built over hundreds of years by various rulers and dynasties. Many of these periods China broke into many smaller countries led by local warlords, in which defending enemy from both sides of the walls became necessary. In the image you can find stairs and convenience doors built from the ground level letting soldiers to get on top of the wall immediately. Where those easy entrances are there are more arrow holes facing inside to prevent intruders to climb up the wall using those stairs and gates from the inside. Near the top of the image where the great wall goes steep up, there are three tiers of wall with three arrow holes facing the walk way defending the signal towers. The base wall is relatively short above the ground in certain areas of the great wall. The fact these walk way facing arrow holes exist indicates the fact enemy's did manage to climb over the walls.
The crucial reason the Great Wall was built is not defending the full scale invasion foot soldiers but the highly mobile cavalry raiders that attacked the smaller towns and villages and escaped before the larger army came to protect them. Which is why the wall is not always tall.
The Great Wall is not one single wall but many layers of walls built over a long periods of time from 221BCE to 1644CE. From mud wall to stone and eventually brick wall. Sometimes it's very hard to say which side of the wall is outside vs inside.
1，Observation Tower (敌台): This is a crucial defense structure on the Great Wall. Soldiers could observe enemy movements and communicate using signals. It also served as a small defensive fortification, with holes for shooting arrows and throwing rocks.
2，Signal Fire Platform (烽火台): Platforms used to signal approaching enemies with fire and smoke. These signals could be relayed across long distances
3， Ramparts (墙垛): Defensive walls where soldiers could stand to watch for enemies or defend during attacks. These also had openings (shooting holes) to fire arrows.
4. Watchtower (箭楼): Larger structures that provide better observation and defense capabilities. Often built on higher ground for a broader view of the surrounding area.
5. Fortress Gate (关口): Strategic entrance points through which soldiers could enter and exit. Often heavily guarded and fortified with defensive elements.
6. Drainage Holes (排水口): Small holes built into the wall to drain water and prevent damage to the structure.
7. Shooting Holes (望孔/射孔): Openings in the walls where archers could shoot enemies while staying protected.
8. Battlement Stairs (登城步道): Stairs built into the wall to allow soldiers to move quickly between different levels.
9. Slope (坡道): A sloped pathway, often used for transporting supplies and soldiers up the steep sections of the wall.
10. Water Drainage System (排水系统): Designed to prevent water from pooling on the wall and causing structural damage.
11. Crenellations (垛口): These are the alternating high and low sections along the top of the wall, providing cover for soldiers.
12. Thunder Stone Shaft (雷石槽): A trough or groove built into the wall used for dropping thunder stones
13. Barrier Wall (障墙): Multiple layers of protective wall with arrow holes that keep the troops on top of the great wall safe where the pathway is too steep and exposed to enemy.
