Like Roman de la Rose, already briefly discussed here, this strip could be seen as symbolic of sexual penetration. The...little green man is pointing out that the fortress (read: the maiden) he has been besieging (read: wooing) has left its gate open (read: gone a-Maying in the fair meadow just as the sun reaches over the hill...a hey nonny hey nonny ho), and its guardian (read: the maiden's father) is helpless to stop the little green man from invading it (read: ravishing her). The "SPLAT" sound effect is meant to signal the intervention of the wind (read: God), which accidentally (read: via divine intervention) prevents the fall (read: the rape) of the stout fortress (read: the virgin). Note that the little green man's clothing is dishevelled in the final panel. Clearly, the "wind" has interrupted him in the throes of passion...I mean battle frenzy.
There is still no real reason for this strip to exist, especially since the type of allegory at evidence here has been out of vogue for at least five hundred years, but at least it's not such an utter waste of space as it at first seems.***
*"Crock" has Germanic roots. I have not the faintest idea whether it is, in essence, masculine or feminine. Let us just assume I am making a glib, silly reference and leave it at that.
**They were originally tried on typewriters, but they kept flinging their poo and jamming up the keys.
***If all else fails, we can use it to clean up the monkey poo.