Birdofwisdom provided decent information on 'Watchtowers' above, and that may very well be what you are referring to. However, there could be some alternatives. Such as:
The Shijin within Chinese mythology. The Shijin are four spiritual creatures which guard each direction of the compass, North, East, South and West. The creatures are the tortoise, white tiger, phoenix and dragon. However, you mentioned an eagle and crow (although you said the unicorn is sometimes replaced by a tiger or similar felines). The Shijin are also alike to the Buddhist concept of the ''Four Heavenly Kings''.
On the other hand, I got little results when I researched the phoenix, eagle, crow and unicorn together, even when replaced with the alternatives you mentioned. In this case, I have little idea to exactly what the 'four guardians of protection' are.
However, providing the symbolism of each animal may help you:
The phoenix is used widely in Chinese folklore and alchemy. To the Chinese, this mythological bird is represents with good fortune, opportunity, and luck. It also conveys to us strength and resilience in the symbolic rising from its own ashes and soaring to greater heights. The Phoenix is said to be a 'Yang' symbol and is associated with fire energies and the four winds. As a point of focus within spirituality, the Phoenix is believed to evoke in us the five human qualities of good cosmic energy which are virtue, alignment, duty, loyalty and compassion.
In alchemy, the phoenix symbolizes the alchemical process of death and rebirth- it represents the purifying and transforming fire. It reminds us of our ability to regenerate, and our power within transformation.
''The lion animal alchemy symbol is held in the highest regards of the tradition. So much so that it symbolizes gold or the sun- the very highest characteristics of ascension and enlightenment. Furthermore, due to its virility, strength, and perceived courage, the lion was seen as the ultimate protector of alchemical secrets, as well as keeper of the underworld.
It's interesting that the lion is considered by many ancient cultures to be a solar animal symbol, however it is primarily a nocturnal creature, conducting its hunting activity mostly at night. Further, the lioness is considered a lunar animal.
This serves as a symbolic message of balance and sound judgment. In that the lion shares the world of both night and day, the lion bears a message of prudence to us. In other words, the lion asks us to not overdo in certain areas of our lives. Rather, keep an even mind and an overall balance in our life activities.''
In general, the lion represents courage, strength, justice, wisdom, power and dignity.
I find the crow such an interesting, and most of all, misunderstood animal. Highly enthusiastic, and a natural charmer.
''Everyone recognizes the Crow's easy energy, and everyone turns to the Crow for his/her ideas and opinions. This is because the Crow is both idealistic and diplomatic and is quite ingenious. In nurturing environments this animal symbol is easy-going, can be romantic, and soft-spoken. Further, the crow can be quite patient, and intuitive in relationships. Left to his/her own devices, the Crow is believed to be demanding, inconsistent, vindictive, and abrasive.''
They do what they want, when they want and they'll blatantly tell you to "shove it" if you don't like it! You've got to admire that kind of gumption and bluntness. As Celtic animal symbols, the crow stands for individuality with blatant disregard to what others think.
''The crow is deeply honored by the Celts as an augury oracle. Crows were rumored to carry big secrets stuffed betwixt their black feathers. Celts knew this, and were wise to let them have their way (indeed, killing a crow was a felony under traditional Druidic rule).''
As we all know, the unicorn is purely a mythological creature. To me, the unicorn represents innocence, wonder, prosperity and purity.
The unicorn is a proud, noble, powerful, fiercely courageous creature who is also gentle, beneficent to his fellow creatures, and happily serene in the end. The unicorn can be seen in art throughout time and it has come to symbolize many things, both Christian and secular.
Unfortunately, I know little else on the unicorn.
For information about the lion and crow (parts in quotes), I used whats-your-sign.com, a fantastic animal symbolism site, as a source/reference.