I know about nine different wickstretchers: five orthogonals - 4c/74, 2c/22, 2c/8, 2c/4, 2c/4, and four diagonals - 2c/14, 2c/18, 2c/18, 2c/48 (some of them in form of their predecessor, including some pentaselah reactions).
They can sometimes appear during evolution of random patterns (usually diagonal ones do that). For example, the second diagonal wickstretcher from the image above briefly appears during growth of the monstrosity, in generation 32971, but bumps into another part of the pattern in generation 34360. Fourth diagonal wickstretcher also appears, around generation 37420, but is also destroyed around generation 39230.
Also, some wickstretchers can be made wider or thinner:
This 6-cell pattern has the smallest population from all patterns in Castles that grow infinitely. After 1 tick, it has the smallest bounding box (3x5) of all such patterns. It evolves into two orthogonal 2c/22 wickstretchers:
Fuses can also be constructed. They are the opposite of infinite growth, but fit in here thematically. Eight fuses are shown, burning orthogonally at c, c, 2c/13, 2c/15, 8c/84, 6c/74, 2c/53, and diagonally at 2c/7. All can be stabilized in a way that leaves population of 0 at the end (they are all clean fuses).
Combining wickstretchers with fuses results in growing spaceships. First example is the fastest wickstretcher with the fastest non-lightspeed fuse; second example has a frontend and backend with well matched speeds, resulting in a very slow growth rate of 48 cells every 814 generations.
This is not quite a true sawtooth, but the population graph does show sawtooth-like properties... it's just the population is always growing. In the slow-growing phase, it raises by 14 cells every 12 generations; in the fast-growing phase, it raises by 30 cells every 12 generations. Every next phase is about 3x longer than the previous one.
A spacefiller that grows linearly is possible in Castles. Smallest predecessors in terms of bounding box (6x6) and population (12) are shown. Notice that the second predecessor uses two pentaselahs.
Population of this pattern approaches the line p=4t, because four lightspeed signals (marked here as S5) that travel along square borders create on average a cell every turn, and the reaction on the corner adds a constant delay, which becomes more and more insignificant over time. At generation 1E+9, population is about 3.993E+9.
Quadratic growth
No perturbation from behind can affect the 2c/4 wickstretchers; because of that, another reaction can continuously send lightspeed "shoots" at them that fill one quarter of the plane (can be easily mirrored to cover one half of the plane, not sure about a full spacefiller).
19-cell predecessor produces this object after about 700 generations, in a slightly messy way. A cleaner version, which consists of 48 cells and grows quadratically from the very beginning, is shown as well.
Shorter side (of the cleaner version) grows with speed of 2c/56, and longer with 2c/4, so the bounding box grows at the rate of t256. It's hard to say how much of this bounding box is covered, but it's definitely more than 12, so population grows faster than t2112. At generation 1M, population is about 1.286E+10, so it looks like the actual growth is about t277.