You can shoot arrows and bolts through the cage against half or three-quarter cover
Projectiles that are very narrow, like blogun needles should of work. Sling bullets would be too wide. You should be able to shoot into the cage with arrows and crossbow bolts:
Medieval arrowheads range in width from 1 - 2.5 cm (about 0.4 to 1 inch). In particular, armor piercing arrows used in combat tend to have narrow heads, of about 0.4 inch at their widest point. The game does not model the mechanics finely enough to disambiguate between flight arrows and armor-piercing arrows, but given that the player characters typically engage more in combat against armored enemies than in hunting where wider-headed hunting arrows would be preferrable, the PCs likely buy arrows that would fit through the forcecage's spaces.
The situation for crossbow bolts appears similar,with armor piercing ones having widths of less than half an inch, so they likewise should be possible to shoot into the cage.
Spells. You also should be able to shoot in with spells like fire bolt, ray of frost or eldritch blast, which do not define a width of their projectiles (and in the case of fire or frost also might be flexible in shape), but of course, this may be up to the discretion of the DM.
The bars of forcecage are as wide as the spaces in between, so they cover half of the caged creatures body if there are no crossbars, or three quarters if there are crossbars spaced the same distance. The spell unfortunately does not state how the bars are arranged, so it is up to the DM to decide how much cover they afford. Without crossbars, or wider spaced crossbars, half cover:
A target has half cover if an obstacle blocks at least half of its body.
With even spaced crossbars, three quarters cover:
A target has three-quarters cover if about three-quarters of it is covered by an obstacle.
Only weapons less than half an inch wide will be able to attack targets in the cage. Even a narrow piercing weapon like a rapier would not fit in any orientation, as the blade of a medieval rapier such as used in the game would be wider than half an inch, and the same goes for the stiletto knife (which also may be too short to reach the target). That means melee weapons will not work normally.
In case there are no narrow crossbars, and for bladed weapons, you could stab with the blade aligned so it slides between with the narrow side, but that very likely should create adverse conditions, as you cannot use your normal movement pattern, so I'd assume this happens with Disadvantage.